Some people like to use shorthands that are common in DC. This is why you'll hear people enunciate POTUS as "POTE-US". Additionally you will occasionally hear people refer to Obama as "44"... if you are looking for the merger of these two, see President Obama's new twitter handle: https://twitter.com/potus44

People on internet message boards like to use lingo. I don't know how many times I've seen people use journalism lingo on HN, dek, graf, kicker, etc. It's not usually necessary but I think it makes people feel cool or send some signal...

I suspect as a follow-on from the days of "Bush 43", in which case it made at least a little sense to say Bush 41 and Bush 43, and not leave the listener wondering whether he had missed an "H.".