The inadvertent reset doesn't seem likely given that you're still seeing a figure for mpg. When you reset, it goes to zero until you start moving again.

I haven't noticed this kind of thing happen, but I've grown suspicious of the navi for mileage. Over my first 10 tanks, the navi mpg was an average of 4.6% (~1.5 mpg) higher than my calculated mpg. A little too much error for my taste. I suppose the odometer could be off - it's usually 15-20 miles less than what the navi shows - but I'm more inclined to think the navi is off.

If I had to guess, I'd say that since it can't maintain a constant link to your car due to tunnels, bridges, garages and such, it maybe tries to fill in the gaps and doesn't do a perfect job of it. The underground garage where I work is the most likely culprit. I drive around in circles everyday looking for a parking spot and the satellite can't see me while I'm doing this.

Anyway, bit of a side track. Since I've lost confidence in the navi figures somewhat, your experience doesn't surprise me. Maybe the satellite lost contact with your car for some reason and when it reconnected, got a different reading from the gas tank?