Turnigy 2S2P Nano-Tech 65-130C issue

[HOTSHOT III]

I have 6 of these batteries which were purchased new about a month ago and they all seem to perform well on the track (used in an HPI Vorza, Hobbywing WP SC8 ESC and 2700kv motor) but when charging one of them won't balance.

One cell goes slightly above 4.20v and one remains slightly below. According to the charger (Core RC UAC-50 which has never given me any real trouble) the cells are 4.23v and 4.18v, I have a Skytronic (chinese) multimeter that measures them at the same voltages but my Draper multimeter that i've had for years says they're 4.18v and 4.11v respectively (all these measurements are when the charger has been sitting at 0.1-0.2amps trying to balance the pack for 15-20 minutes, i.e at the end of a balance charge).

I must say out of the 3 devices used to measure the cell voltages, i'm inclined to believe the Draper multimeter. The pack isn't getting hot when the dominant cell reads more than 4.20v, all that happens is the charger won't finish charging like it does with all the other packs (although I have a couple of these UAC50s and they are a bit sensitive when balance charging, sometimes they take longer than others).

Does anyone have any suggestions? As I say I can't detect any performance difference in use between this pack and the others so could it be that a difference between cell voltages of 0.05-0.07v is not really a problem, even with a brand new battery?