page of orange book

Ok I'm a pharmacy tech, and there is something that really confuses me. How can two brand names, Synthroid and Levoxyl, be both equivalent to say levothyroxine Mylan brand, but not be equivalent to each other? This seems to fly in the face of logic, like if a = b= c, then a =c is also true.

I think this has something to do with bioequivalence, meaning that Synthroid and Levoxyl are different enough from each other to be separate, but both are within boundaries for the mylan generic?

Another example: Proair, Ventolin, Proair. all albuterol HFA, apparently not equivalent. So if rx says albuterol HFA, we can dispense any of the above. Albuterol HFA = (a,b,c) but a b c are not equal to each other? is this due to it not being tested? or what the heck?

Using logic, none of this is comprehensible.