Someone, after the fact, sat down and went "hmm it seems to have better firepower than a regular TIE - what if it had its weapons upgraded?"
Which is pretty well accounted for with the 33% increase in firepower it has over the Tie/Ln or Tie/Ad
This ^ ^ ^ A thousand times this. If your point is "I don't like 33%, I wanted to see a 75% increase, why isn't the attack power of an Interceptor FIVE or something" the simple answer is: it would bone the game balance. Hard. So it's never going to happen. I apologize if that fact haunts you and keeps you up at night. ![]()
I don't even know how to convey to you the other point, which is that canon is slave to the needs of the narrative, and not the other way around. It is tweaked, cheated, or straight up ignored any time it's convenient to the needs of the writer/director.
If you honestly don't get that and think that they're handcuffed to canon, sitting in some writer's room going "curse my fate! I wanted Luke to survive this scene, but he was hit with a 9.3 laser cannon, and we all know that is a guaranteed kill. Why god? Why???" and not just "ok, so Luke gets winged by the Interceptor, we see some sparks and stuff, he shouts some technical nonsense at R2 about locking it down, next scene" I don't know what more I can tell you.
Made up. As needed. For the story.