Green is "Main St" and the rest is "Main Street"
First of all, I am amused at just how many "Main" streets there are in the US. But also it looks like the name expansion bot was only set loose west of the Mississippi. The bot was run long before I joined OSM so I was not around for any discussions about why this was done. I'm sure it is buried somewhere in the mailing list archives...
I haven't done any substantial editing east of the Mississippi which explains why I hadn't noticed abbreviated street names before. I'm still not sure why I found some in western Kansas. The most likely explanation is that a few of the bot uploads failed for some reason, leaving pockets of abbreviations around.
I pinged balrog-kun about it on IRC so the mystery may be solved in a few hours but I thought the image was amusing enough to share anyway. And for a few more giggles, here is the whole world:
It seems the UK has a fair share of Main streets as well, plus a few scattered mostly in former British colonies. I'm sure they started it and then I guess we here in the US just turned it up to 11.
Update After talking to balrog-kun: Apparently he was moving west-to-east and at some point some people started complaining for some reason so he stopped. Fair enough I guess. He's not sure about the ones I found in western Kansas though.