When my batteries are very low and the Magnum is "maxed out" at 125A charging, the BMK shunt also indicates roughly the same going into the batteries. The BMK's number is offset by DC loads that the Magnum is powering in addition to charging the batteries, so it was off by a few amps but not that many.
In the photo above, is it possible that 20 amps were going someplace else in the trailer? That's quite a discrepancy.
I wonder what the Magnum's charging output looks like on a scope, if there is modulation to it that might affect the Trimetric?
Way too cold for me to play in my trailer, so I can't experiment on anything right now.