I've seen a few online services being hosted on machines named "na1" followed by the name of the company... Things like na1.replicon.com or na1.salesforce.com. Maybe it's just one of those days by why "na1"? Why not "app1" or something more meaningful?
Anyhow, just the weird question of the day. There is more than likely a very good reason for the choice of those two letters but it just does not come to my mind.
Seems like the answer was indeed dumb obvious (after looking for potential meaning of the abbreviation NA). In the two sites I stumbled across NA seems to mean North American. They also have a EU site which would lead me to think it's to distribute the applications geographically.