Ok I've been playing this game for a couple of years now and I still can't figure this out...

Boss fight comes up, comes round to finishing them off and two numbers pop up on my screen in red - one with a -negative number and one with a positive number and for the life of me, I can't figure out what they mean.

I've tried to calculate the 'overkill' amount of damage (boss has 200hp and I hit for 1k for example) - doesn't match up.

The actual damage I do (1k) vs the amount of hp the boss has left (1k minus 200) - doesn't match up.

I just can't see any correlation between what I do compared to what happens - anyone got an idea?