Assuming no air is entering the reservoir from a faulty or improperly seated seal, your problem could be related to the heating of the water.
It might be overfilling the pan because as the water in the reservoir heats, it expands/pressurizes and forces out more water than the "vaccuum" in the reservoir can counter.
The same thing happens with a hummingbird feeder in the sun--it heats up and the nectar leaks out, whereas in the shade it does not (although, granted they always leak on a day that starts out cool and then heats up).
I've never set my water on a particularly level surface, and it still doesn't fill to the brim--even on the days that climbed into the upper 90's and and beyond. You have to tilt it quite a ways before an air bubble can get in to relieve the pressure and allow more water out.
All bets are off, though, if the font is being jostled enough to allow the water to forcefully escape through the spout.