Thought about this for a while.
At 90 Watts load, the Multiplus 5000 is operating at around 80% inverter efficency. So, sustaining a 90W Load requires about 112,5W DC Discharge.
That brings the actual vanishing difference down to 81,5Watts (126 + 68 - 112,5)
Within that 81,5Watts, there are consumption of the Mppt, losses due to cabling, measuring imprecisions, measurement timing offsets - and eventually drain caused by reactive power.
For the later issue, there is a topic created yesterday, where it is asked if reactive power is causing a DC-Discharge or not, see:
Nut sure if all that is enough to sum up to 81 Watts, but the bottom line is, that you will never see the values adding up perfectly, when passing the AC/DC border.