Most likely the percentage of planes photographed in the u.s.
I think the most frequently uploaded locations are the UK/Scotland and Australia, followed by the U.S. because U.S. airports' authorities have such an anal attitude towards spotters/photographers and this country is full of Nuts. They close all the Observation Decks because they're afraid some nut is going to try and commit suicide by jumping off....heck, now we even have to leave our mailboxes open so the Postman can see there is no bomb in it!
You can't tell by reg #'s because like Jwenting said, many are leased & owned by companies in different countries.
And U.S. built versus other countries of manufacture will surely start another Boeing v Airbus war here!!!!!!!
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens