Challenger 2 TES armor is broken/M392A2 vs M728

So, I have to questions to ask:

Why are Challenger 2 TES’s side armor plates as effective as the ones on Challenger 2 (2F) despite being much thicker? And why is Challenger 2’s armor so bad overall? Shouldn’t it be balanced as this tank is already pretty weak and nothing at top BR is historical anyway?

Is there some bug with 105mm APDS or does 10mm penetration really make such a difference?