Charger Voltage Issue...Anyone else had this?

[Cremegg]

I have the Overlander RC-6S Charger and use it for 2s batteries

If I charge on 0.1a it shows the cells as balanced but if I charge at 1C (4000) the voltages show 4.20 and 4.05, it still charges the cell up at the end but I can't figure it out why they are so far apart on a higher amperage

Can anyone shed some light upon this?

Thanks