[chuckfourth]

Hi Nox
You ask
"Also, why should tank benefit in game against all guns when FH only gave benefit to some ammunition fired by the same gun?"

As you correctly point out most FH armour is thin, in the roughly first half of the war before the advent of the ammunition types and calibres that had improved penetration against FH armour. Giving FH armour +1 doesnt disadvantage any guns.
As the shells and calibres that have improved penetration values against FH armour begin to appear, in the vast majority of cases the penetration value of the weapon is so much greater than what the thin FH armour defence value is, it hardly matters +1 or not, while against the remaining, majority of shells/calibres modeling becomes more accurate with +1.
But to answer you question specifically,
because
Currently all FH armour is disadvantaged against most regular ammunition types. This is incorrect. As Ive tried to explain above it is less incorrect to give FH armour +1 than to give it the same value of RHA, which is also "wrong" because against most ammunition types FHA performs better than RHA.
For your example,
"6-pdr British gun firing APCBC penetrated more FH than RHA but same gun firing AP penetrated more RHA than FH. Why should British gun be punished?"
I would ask
Why should the 6 pdr AP shell be given bonus penetration against FH armour as currently modeled?

Best Regards Chuck