The same damage (or about the same)? My best educated guess is that you're calling a "damage" routine or something that doesn't know what it is damaging (a generic "@DAMAGE" variable or something, then it checks to see if DAMAGE > 0 and if so, damage whatever)

Or it could be something like this:

-Your Turn, select attack
-Randomize Damage (into variable HPDamage)
-Monster HP = Monster HP - HPDamage
-Monster's turn, might not be doing anything damaging but HPDamage is still there
-Your HP = Your HP - HPDamage