LongMon - Cell Voltage measurement - toleranc problem, or not?!

I have in my Setup 80pcs. LongMon (48V 16S 5P LiFePO4) on one WatchMon-CORE.

Now a question regards the Cell Voltage measurement. In my Stetup i have a overall differenc on all 80 cells from around 30mV showen in the WatchMon-CORE.
But in reality - when i measure everything with an Fluke Multimeter i got only 5mV over all differenc.

So the LongMon´s are clearly out of callibration somehow.

When i do a calibration of the LongMon - is the calibration correction value stored in the WatchMon ore in the LongMon itself?

Or is this tolerance (30mV) ok?
How precise are the LongMon´s with the Voltage measurement?