Okay, but this still does not merit a random % boost from designation. I see no reason you couldn't then designate an SPG as "anti-tank" and just have your AT divisions suck at piercing if you want.
Now I'm confused. Why would you try to designate something with crap piercing as a TD just to harvest a percentage bonus to piercing? It only makes sense to designate AFVs with the right kinds of guns as TDs anyway. For example, rocket modules have worse piercing than Tier 2 infantry kits. Why would you try to scam the system to get a 30% boost to piercing when it would virtually no impact.
Say what you want about percent increases to unit stats in general game design, but in this case it means that it only makes sense to designate AFVs with real potential to kill opposing tanks as TDs.
This does not explain why tank piercing/hard attack is higher than AT piercing/hard attack, however.
Because we don't have an artillery designer to give AT, ART, and AA more granularity, so it's almost impossible to make them uniform? Between squeeze bores and the super-heavy cannon in the super-heavy tank tech, even if the base stats were uniform between high-velocity guns and AT guns, there would be no uniformity in actual practice.
I've also had some people argue that the TDs deserve better piercing or hard attack due to better mobility. I'm not the expert on this stuff, so I'm not sure.
For what it's worth, I suspect that regardless of historical considerations, there's a game design incentive to make TDs a bit better at piercing as an offset to their price increase over AT guns.
As for guns mounted on TD, it would be a more reasonable abstraction to match ammo to turret type than it would be based on designation alone,
Why? Vehicles functioning as tank destroyers did not have uniform fixed mounts.
Just off the top of my head, I can think of the M18 Hellcat and the M36. Both had turrets, and both were intended to focus on killing actual tanks.