Yeah, that's not unheard of. You need to bear in mind that the electronics in the charger are as cheap as the manufacturers can get away with (for the most part) and we're all relying on them to perform pretty accurate measurements so there's bound to be a difference between units.
If I just get a bit technical for a second:
The most common processors in these chargers are Atmel AT Mega. The ADC (analogue to digital convertor) is 10 bit (1024 step) and they operate from 3.3 or 5V.
If we assume that it uses 5V reference then each bit is 5mV, i.e. the smallest change you could theoretically detect, so you can see that 50mV could be easy to lose.
I hope that makes sense!
Ian
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