It could be a corner room that's a little larger, it could be a suite or mini suite, it could be an accessible room they couldn't sell. It could be a beach property and be a room that faces the ocean.

I've even seen one property where I swear they were giving and selling upgrades in name only to make people feel special or collect some cash, I had stayed there previously and gotten an identical room (though on the ground floor) that was not considered an upgrade.

It could be just about anything.

If you click on accomondations on the website for that particular property, it will list (usually) the types of room(s) available.