I was watching DVC Mike's video on Bay Lake Tower and one shot is of the floors in the elevator I noticed that there is no 13th floor. Some one must be superstitious. Not sure if this was brought up before.
Generally, this is just done for marketing, IMHO. Honestly, they get to build a whole less floor, but still get to promote the building as 'X' number of floors, when it's really 'X'-1. It's a smart marketing thing, and no one cares.