My heroes have maxed skill, but in battle stats there are at 70%. Why ?

I'm not sure which class it is, but either farmer or trader have a penalty of -30% when raiding. This is most likely why you see the 70%... You do get a chance to change your class at certain castle levels... one is 19-20. Raider class has bonuses to raiding as well as it's class legion being able to raid gold.