When I am at, say, 200 heatlh, and I cast a 400 heal on myself, I sometimes end up wtih 599, not 600. Same when I have taken damage. I'll sometimes end up with more health then I should, or less. It's bothersome, because there are times when 1 health point makes a huge difference.
Another thing that I've run into a lot lately is the monsters health. I recently spent a day playing, where I ended up, probably by chance, defeating a lot of monsters with the exact health they had. A few of these were very close battles (one was a final boss mob battle). The monster had, say 60 health points. I choose one of what I call my freebie cards, a wand attack that does damage for no pips. The attack does exactly 60 points damage. Yet instead of that monster dying, I'm faced with having to cast yet another spell on it. It's frustrating. In one case, the mob then regenerated points and lost no pips, defeating me, when I had, in fact, defeated it to zero health.
We could use a bit more detail on the circumstances here. Are the values you are talking about taking damage type into account?
If you hit a life monster with a life spell it does less damage than if you hit them with another damage type. A life monster is naturally resistant to life attacks. Alternately they're weak against death magic so death spells will do more than the stated damage. This also works in reverse.
I am talking about actual damage done. Where you can see that the damage done to you is, for ease of use, 99. You started with 1000 hit points. So you should have 901, correct? Yet there are times I end up with 902, or 900.
And yesterday, while in Marleybone on my Storm wizard, someone I was grouped wtih had low hit points left. He got attacked by one of the clockworks, and had 0 hit points left. Next round though, he had 1 hit point. It is the same thing that happens to monsters. You can hit them, and they have 0 hit points. You think great, you won, then they magically are still in the game, and have 1 hit point next round.
Which makes it hard when they go first, and you are low, and you think you've won, and surprise. You end up being defeated because this monster that had zero hit points suddenly ends up with 1, and can attack you.
I'm not talking about a life card hitting a life monster for less damage. I'm talking about the actual damage done, the red number you see floating upwards and disappearing. Not what the card says it will do, which is usually a range.
This is a known bug with damage calculation. One effect of the bug is what you described: if you defeat a foe with very near the exact points needed, it will occasionally die but remain in battle, regain a point of health on the next turn, and have to be defeated again.
The KI developers are aware of the problem and as of a couple weeks ago had not yet found a solution. While it is annoying to have a foe come back to life, most of the times when health is off by a point or two it is not at the moment of defeat and has little impact on the game. That may keep it from being at the top of the priority list. In the meantime, think of it as a strange side effect of using magic.
Damage calculation being off by a point or two is currently fairly common in the game. It affects both monsters and wizards. It affects both PvE and PvP. It affects many more turns than just the one where a foe is defeated, but it's difficult to detect during the rest of the battle--how often are you certain exactly how many points of damage will be done, and aware of the foe's before-and-after scores, so you would notice if they were off by one or two points? It is easier to notice in your own health-- during battle you have both a red crystal ball and a parchment tag displaying your health. These two displays differ by a point a two fairly often, but we hardly ever notice because we habitually look at only one of the displays.