Perhaps it was due to charge efficiency? The Trimetric uses 94% for 6% average heat losses across the SOC range . ISTR it was posted here that Bogart recommends using 99% with LFP? So if the Xantrex one is like that, it would get out of whack with the LFP.
Eg, when the LFP has taken 100% of what it was down, and 1% of that was heat, the monitor should show AH restored as 99. With the FLA, it would show 94% restored.
The Trimetric has a way to set the charging efficiency factor to something else instead of 94 if you find it is not right for your set-up