Because there are several.

Update: The following have been fixed:

• Iori's description
• Athena's ES
• Kyo's everything

Athena Asamiya 7*

• Athena's ES damage reflect has a proc rate of 0%:

          {
              "conditions": [], 
              "dmg% reflect chance%": 0, 
              "dmg% reflect high": 50, 
              "dmg% reflect low": 50, 
              "passive id": "26", 
              "passive target": "self"
          }
  

• Damage reflect does not stack. Each source will attempt to proc and calculate the results individually, so you cannot force this ES to proc by adding other proc chances. It simply doesn't function.

Kyo Kusanagi 7*

Pretty much completely screwed up and literally a hindrance to the player. Kyo was actually fixed before release, but seems to have reverted to this broken state.

• His Injury and Weakness are inflicted on the player squad

                  {
                      "effect delay time(ms)/frame": "0.0/0", 
                      "injury%": 80, 
                      "proc id": "11", 
                      "target area": "aoe", 
                      "target type": "party", 
                      "weaken%": 80
                  }
  

• His spark damage is actually spark vuln, targetting himself:

  • BB

                {
                    "effect delay time(ms)/frame": "0.0/0", 
                    "proc id": "58", 
                    "spark dmg received apply%": 100, 
                    "spark dmg received debuff turns (94)": 3, 
                    "spark dmg% received": 80, 
                    "target area": "aoe", 
                    "target type": "self"
                },
    

  • SBB

                {
                    "effect delay time(ms)/frame": "0.0/0", 
                    "proc id": "58", 
                    "spark dmg received apply%": 100, 
                    "spark dmg received debuff turns (94)": 3, 
                    "spark dmg% received": 90, 
                    "target area": "aoe", 
                    "target type": "self"
                }, 
    

  • UBB

                {
                    "effect delay time(ms)/frame": "0.0/0", 
                    "proc id": "58", 
                    "spark dmg received apply%": 100, 
                    "spark dmg received debuff turns (94)": 3, 
                    "spark dmg% received": 90, 
                    "target area": "aoe", 
                    "target type": "self"
                },
    

• And finally his UBB hit count buff doesn't seem to apply correctly to other player units:

                  {
                      "effect delay time(ms)/frame": "0.0/0", 
                      "hit increase buff turns (50)": 3, 
                      "hit increase/hit": 3, 
                      "proc id": "26", 
                      "target area": "aoe", 
                      "target type": "self"
                  }, 
  

Iori Yagami OE

Iori's ATK passive SP enhancements are incorrectly implemented, judging by how other passives function.

They currently look like this:

• 50% ATK:

          "id": "140001", 
          "skill": {
              "bp": 20, 
              "desc": "50% boost to Atk", 
              "effects": [
                  {
                      "passive": {
                          "atk% buff": 50, 
                          "passive id": "1"
                      }
                  }
              ], 
              "id": "140001", 
              "level": 1, 
              "name": "50% boost to Atk", 
              "series": "100000"
          }
  

• 80% ATK:

          "id": "1000000046", 
          "skill": {
              "bp": 10, 
              "desc": "Boosts 50% Atk to 80% Atk", 
              "effects": [
                  {
                      "passive": {
                          "atk% buff": 30, 
                          "passive id": "1"
                      }
                  }
              ], 
              "id": "1000000046", 
              "level": 2, 
              "name": "Boosts 50% Atk to 80% Atk", 
              "series": "1000000046"
          }
  

As demonstrated by the data of several other units, including most importantly Zekt (due to having an improving passive with the same ID), the system does not add SP enhancements together to determine the bonus they grant. It either gives the best value, or the last one in dependency order within the series (though it's not exactly clear which).

This means that only Iori's first ATK passive is functioning as intended, and the second passive, which is supposed to increase his ATK bonus from 50% to 80%, either does nothing, or reduces his ATK bonus to 30%. If they do work, it's an incredibly janky implementation (and the only reason it could work would instead mean his 100% BB ATK passive actually gives 150% total)

Alim's usual implementation, shown most directly by Zekt, is to have all buffs to a single ability within the same "series", but with different IDs. Zekt's in particular show that the ID of the passive itself isn't too important as long as the series ID matches (all of his HP passives are in series 40000, but 20% HP is ID 41000, 30% HP is ID 40200, and 50% HP is ID 40500), and the highest value is the one that's used by the game with all others ignored.

Additionally, Iori's LS has an incorrect description which implies an 80% HP boost when the LS actually provides 40% HP:

• "80% boost to Atk, max HP, hugely boosts BB Atk, enormously boosts Atk when HP is low & greatly boosts BB gauge when damage taken has exceeded certain amount"

Credits to /u/Academic_Dragon for informing me on that one.

These are just the bugs, but the community generally feels that many of the KoF units are poorly designed for the global OE meta, being either exceptionally niche or just plain not competing, and this has probably driven some summoners away from trying to acquire these units. That should probably be its own discussion later on, though.