Why does fuel flow go UP when leaning on the ground?

A hysteresis loop in this context refers to the idea of a lag between
cause and effect. The mixture control cable is not going to accurately
100% of the time precisely represent your mixture knob movements. Slack
and stretching in the system can make it such that, for instance, when
you change the direction you are turning the knob, the actual mixture
setting on the carb (or equiv for FI systems) might not move exactly
the same at the same time. Imagine if the cable was very sticky for
instance. You're not going to get ideal response from your inputs.

As for whether or not that's your reason for increased fuel flow, I'm
on the fence. I like the increased revs theory too...

-Ferdi