I've noticed this for a while, but wasn't quit sure (its one of those things that you need to slow down and pay attention to figure out) but the damage that floats of a mob when hit with PH that does multiple kinds of damage does not reflect the damage reported in the "action box" and so I am not sure what damage is being inflicted:
Example, I hit with four kinds of damage Pierce=X, Cold 20%X, Psionic 10%X and Lifesteal 20%X
The action box on the right reports all 4 types of damage, but the number that floats of the mob is only X+20%X.
I don't know if the actual damage inflicted is that which is reported on the right or the number that floats of off the mob (it seems to me based on how many hits it takes to kill a mob that most likely the smaller number?), but it should be the same as the right, no?

But Lifesteal is supposed to do hp damage and is not counted...

done- (sorry it appears twice, because I didn't realize that the first one had been posted-not sure how to delete them)...

You are taking the mob's resistances into account?

For example in your first post you say:

I hit with four kinds of damage Pierce=X, Cold 20%X, Psionic 10%X and Lifesteal 20%X
The action box on the right reports all 4 types of damage, but the number that floats of the mob is only X+20%X.

But does it just happen that the mob's resistance are such that the total damage to him is 120%X?

In the textwin you see, eg,

You hit Mob for 100 (-20) damage with pierce

IIRC that 100 is not actually the total as you might expect. In fact the total is 80. So (in this example we'll assume everything does hp damage only):

You hit Mob for 100 (-50) damage with pierce
You hit Mob for 20 (-20) damage with cold
You hit Mob for 10 (-1) damage with psionic
You hit Mob for 20 (-4) damage with lifesteal

If you don't account for resistances (or think they're already counted in to the non-parenthetical numbers) looks like 150 total damage. In fact it is 75, so this is the number that (correctly but maybe confusingly) appears on the map.

I think that's right anyway.

That's possible/probable.