The ADC is non linear and especially in at the range extremes you should not expect to match accuracy of the raw output. And each individual MCU is different in ADC response, hence I have to manually try to calibrate each to the best of my ability. Still, very likely that 2 CRs will show slightly different OLED readings.
Take the OLED as a cherry on top of the cake, its very useful in most cases and its very convenient, even though if you have a CR you most likely have a basic DMM to read the raw output which is far faster and more precise.
If you have the latest firmware, then the ADC range reference is split in half as described in the latest mk4 release, this is increasing OLED readings resolution as well as overall accuracy.
Recent batches do come with this firmware.