Thats because certain small arms have more penetration then others. but the extra ensures its protected from the 2 AV options the ostheer has : 222 and incendiary rounds.
This unit is still op however having more AT and AI then the Stuart while costing significantly less in fuel and arrives earlier. also doesnt fit in the current ukf and the buff is simply not warranted. the ukf shouldnt have this kind of offensive tools available being a defensive faction. buffing the IS is a far better solution and vicker carriers is.
Incendiary rounds still damage the AEC (especially from behind, which can kill it in the blink of an eye), and the 222 costs less than 3 times the fuel. Also, to get an AEC you have to sink about 455 MP and 65 fuel into a single unit. The Stuart costs 240 MP and 70 fuel after teching, and as shown earlier in the thread the Stuart wipes squads faster while the AEC has to whittle them down. Also, the Stuart's vehicle stun actually works.
Though I do agree that Ostheer don't have that many options against an AEC at the time it comes out. It's a timing issue. I think the AEC's stats are fine.