Now I see where that idea came from to discharge the battery completely, charge it up to full (as many AH as it will take) and then declare that is the true capacity in AH. Don't do that with your Wets or AGMs :(
Whatever, it seems to use AH vs capacity to get the SOC (which it calls "capacity percentage" Grrr.)
It does not relate the cell voltage to SOC, so I am back to just being confused about that. I must read that Li bumph again to see where I got that notion, and have another look at your results.
I don't see how the monitor can have a setting for 100AH as its capacity, run up 125 AH and say the battery is at 100% of its capacity. And by magic, the voltage is 14.4 with amps at 3 like they should be for "full" with a BB.
There is still the mystery why all the distortion was at the low end of SOC in your results.