GOM goes UP when HVAC turned on (!?) and my snow tires are apparently MORE efficient

1. So I've seen some weird GOM behavior lately on my 2020 Bolt now that the weather is cooler (it's my first winter with it). Here's what I've seen: driving along when it's cool or rainy with the HVAC system completely off. Then I press the buttons to turn the upper vent and windshield/defog on together with just the fan, no heat. The GOM will go down for a second, then jump back UP several miles higher than it was before I turned the HVAC on. I can then turn both vents and the HVAC completely off again and the GOM stays at the higher value.

Cool way to get some free miles I guess? I know that I'm not actually gaining any range, I'm just puzzled why the GOM seems to think I am.

Had noticed previously that when you press just the windshield defog button it seems to run the AC compressor and/or heat to dry out the air and GOM goes down. But when you press defog in combination with another vent it just uses cabin air without the AC or heat and the GOM doesn't go down (now mine goes UP). My auto de-fog is off FYI.

2. Also, this week I had my new 205/60R16 X-ICE Snow tires on 16" regular Cruze rims (not Eco) installed. I reset my trip meter when they were installed. I'm getting 4.7mi/kWh in ~40F temps through about 140mi of around town and some highway driving...significantly better than I was getting with the stock tires at similar temps recently. I had kept the stock tires right at 39-41psi cold. I was expecting my efficiency to go down, not up. Are these new snow tires magical? This is non-scientific anecdotal observation, but I'm happy.

For reference, only have ~6600mi on the car