That last five points...
I think the health status of targets only increment/decrement at an estimation rounded to 5hp.
If he had something like 7hp, it might show only 5, and even if you hit him for 6, it would still show 5 because it never displays 0. Also, you have to factor in slight regeneration sometimes... which may cause it to look like you didn't even hit him, even though you did.
It's odd.. but it seems normal to me.
Aye, Master-Blade has it correct. The Target Window only shows HP in increments of 5. So, even with only 1 HP, or even 0.01 HP left, it will still say 5.

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Okay, thanks guys. That makes sense - annoying as heck, but still makes sense.
Global: @All-American Teen
70 toons across 11 servers.
Top hero -All-American Teen lvl 50 eng/invul Tanker (01:10 EST; 1/24/09)
Top villian -Poisoned Plum lvl 30 robots/devices Mastermind
There is also the case when the enemy's regen tick kicks in just before your damage is applied. Combine that with the frequency the enemy's HP is updated with the rounding factor.
This is when I end up using my origin power.
Me: SLICE
Them: Ha, still standing.
Me: Taser Dart
Them: THUD
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As of last, I've noticed that most of the enemy I face seem to have five points of health left unless I do close to a massive attack. No matter what I would hit it with, no matter how much damage I would do, there would more times than not where I would have to hit it one more time just to deal with that last five points and drop it.
In fact, a couple of times after I started noticing this, I took a closer look at how much damage my attacks were doing and how much health the enemy had. There were more than a few times where they had five health left when that last attack should have dropped them. (I was keeping my cursor hovered over the enemy stats, and could tell just how much health any one of them would have.)
Now if this only happened once in a while, I'd be willing to put it up to "Okay, my last attack didn't do enough damage." But when it would happen over and over again, even after landing a massive critical hit, I started getting suspicious.
I've already sent a bug petition ingame, but thought I'd post about this here.
Global: @All-American Teen
70 toons across 11 servers.
Top hero -All-American Teen lvl 50 eng/invul Tanker (01:10 EST; 1/24/09)
Top villian -Poisoned Plum lvl 30 robots/devices Mastermind