Thanks for the reply
So, very simply put, it is because there is a very thin area outside where there are relatively few air molecules striking the surface perpendicular to them (i.e. the aircraft) because they are too busy whizzing by.
This brings me to my next question... If the pressure inside is slightly less than the outside of the aircraft (this is something, myth or otherwise, that has been 'confirmed' to me by numerous people), then why is it much harder to pull the storm window (i.e. on a PA28) to the open position, INTO the lower pressure interior when in flight/at higher speeds?
I'm probably getting many wires crossed here, but it's one of those little mindbenders in aviation that I've decided to sort out once and for all!