In the old HoMM the mechanics of the fights were quite simple and well explained in the manual. In this I understood little, since the heroes have very high attack and defense values!

High Attack increases the damage dealt, High Defense reduces the damage received, similar to previous games in the series.

In HoMM4, heroes can get ridiculously high stats, especially for carry-over heroes in a campaign. In later missions of a campaign, if you've been stacking all the stat buffs on a hero, then they can solo entire armies with relatively little risk.

Now I got it! In HoMM4 heroes stats don't sum to unit stats. Unit get bonus only with the relative skill

I think the stat values are plugged into the formulas in the same way, but hero skills can give the hero a huge boost (like GM Combat boosting Defense to 60).