[At' Pressure] Glass of water held upsidedown

Hey,
I'm sure you are all familiar with the experiment where you take a glass of water , put a some seal on it and then turn it upside down.
Due to the (atmospheric pressure) - (the air pressure in the glass) exerting a force on the seal upwards greater than the [mg] of the water, the seal stays in place.

here's a demonstration - physicscentral.com

What I don't understand is why there's a pressure difference in the first place.
As far as I know - if I seal the cup then the air pressure is the same there as it is outside (with minor difference - ρgh ) what makes it change when I turn it upside down?

Thanks.