You see unbalanced discharge in certain mods, generally either the battery furthest the control board at the tail end of the series or parallel circuit. Some it is the closest battery to the control board I have seen, especially in series mods. Examples batt 1 3.7, batt 2 3.5, batt 3 3.3 or batt 1 3.3, batt 2 3.5, batt 3 3.7. If the batteries are old, 6 even 12 months old or have seen some serious abuse this could 1 to 3 month mark, the lowest battery always if they stay in the same position in the mod is an indicator which is your most worn and weakest battery, if you are staying below 50% their max CDR you'll see this at the 9 to 12 month maybe longer area, if you are constantly at 75% or greater CDR you'll see this in the 6, 3, to 1 month area.
Being 20amp CDR batteries, 50% = 10amp CDR or less always asked, 75% would be in the 10 to 15amp range or higher. My batteries, unless in a mech which are my HB4 30amp domain only, in my regulated mods I use 20 amp batteries which rarely see 10amps or higher, most times they are in the 8 or 7 amp constant power request range, on my Sig150 and IPV3 I see the batt 1 will be at about 3.6 to 3.8 range while batt 2 will be at 3.4 or 3.5 under normal use. In may regulated mods my batteries are pulled for rest before recharge at about 40 to 50% charge on the indicator, my DNA75 mods I've setup for 3.6v cutoff so see recharge when the battery bar is fully almost empty. Is it something to worry about, it can be as it can be a battery condition indicator, but if maintained, newer, and well cared for batteries, most times it is just how the mod operates.
Also remember even with A Bin primary manufacturer batteries like Sony, LG, and Samsung, you could have a weak QC battery from the get go, though it is rare, there in that instance to test is to manually rotate the battery positions each time, cycle 1 - batt A in slot 1, batt B in slot 2, batt C in slot 3, cycle 2 - batt A in slot 2, batt B in slot 3, batt C in slot 1, here would tell you not only if a certain battery is weak, but a baseline of which battery sled slot is doing the most work in your mod to guage things better.
Keeping a battery log book or log spreadsheet/document also help troubleshoot these things a bit as well
1)Battery Label/Number
2)Date Purchased/Aquired
3)Date First Full Charge
4)Number of Recharge Cycles (this is battery life measurement, most battery cycle lives are between 150 to 250 or 300 full charge cycles from 2.5 to 4.2v, half cycles and not charging to full 4.2v everytime in the range of 4.1 to 4.15v you can extend the life cycles out to 300, 500, up to 600 charge cycles (come specialty more expensive chargers allow you to set the full charge cutoff to 4.1 to 4.15).
5)If put into storage the battery charge level, never store batteries at 4.2v for longer than a week, longer storage keep them in the 3.4 to 3.6v range and check them periodically with a multi-meter, goes below 3.4v charge them back up to about 3.6 to 3.7