MPG Drop when Gliding?

I had something interesting happen when driving today and was curious if anyone had some feedback?

I was driving down a long hill between 30 - 40 mph. When the road was steep enough to coast (no-pedal regen), my instantaneous MPG was the expected 100 mpg.

However, when road flattened out a little and I switched to glide, the instant MPG dropped to 50, and worked its way back up to about 75, until I let off the gas pedal and went back to coasting.

It was weird-- why on earth would 0-energy gliding drop my MPG when my engine was warmed up?

Other considerations:
-I'd been driving about 20 minutes on the highway at this point, so the engine was warmed up.
-Battery was fully charged from going down the hill (as in, the standard "full" + 2 more bars)
-AC was off.
-Outside temp was 66 f.

My only theory was maybe the long regen cycle going down the hill had heated up the inverter, so the computer in an attempt to minimize current running through it, switched to gas instead of electric when I started pushing the accelerator in, even to just a glide state.

Any other thoughts?