I've been thinking about your post for a few minutes.
How many sets of batteries are you cycling? And, is it only one set doing this? The possibility exists of a bad battery. Others (and myself included) have reported an even discharge: Both batts being equal voltage when coming out of the mod. Since yours aren't, I'm left wondering a little if its a weak cell and if a different set of batteries might be the answer.
And, worth mentioning - My xtar Wp2 II charges mine to 4.15 and that's exactly what I think should be expected judging by review comments.
I get the exact same with my Pila. I don't drain my VV W batteries all the way but these are my EXACT results so far.