I've seen all of these as well.  Also one I have seen a LOT is when I fire my jav's one at a time at an enemy, the first will hit and do damage.  Then I'll see their health go back up part way in one. Beg chunk, then the second volley hits and does damage.

I've always assumed that it was lag, or perhaps a data sync issue?  

the thing you described happens with em rifles alot

you shoot, you see like -80% in HP , then the bar bounces back to show only like a -40% decrease in HP

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