Brick 1 measurement lower than others

I have been using SimpBMS to log data from Tesla modules. For some reason Brick #1 in every module give 3-4mV lower voltage values than Bricks 2-6.  I have tested this with two different battery packs. I don't see the same phehomenon when I look the same voltage values from Tesla's own diagnostics. Is there an explanation to this or could this be some kind of bug?