Add on:

Tested it also with a Dagger. A +4 D Dagger had 90 P.Atk. and the +1 C Dagger had 89. The P.Atk calculation on Daggers are fine. The +4 D Dagger gave my Character more P atk as the C Dagger. So its actually ONLY Bow`s P.Atk that seems bugged.