I've started painting my Orion so I took a look at the free record sheets here and noticed that the ON1-M is 15 points cheaper than the ON1-K but seems generally better.

I know 15 points is not a lot, but for 15 points less you only lose 3 points of armor on each arm and 1 point of rear center torso armor while upgrading the LRM from 15 to 20, upgrading the AC/10 to an LB 10-X AC, adding the Narc, and going from 10 normal heat sinks to 11 double heat sinks. Obviously they're from different eras, but I thought BV was intended to be balanced across eras and this seems like an overall upgrade plus a savings of 15 points. What is it that I'm missing?

Edit: Thanks for all the answers! I’m a relatively new player and learned more about how armor and heat sinks are calculated in BV and how significant the risk from the extra crit slots of the XL engines are. I still think these two mechs are an interesting comparison but I understand how the -M is not strictly better now.