Its a "traditional superstition" i suppose you could say. The 13th floor is either omitted, re-numbered as 12a, 12b or even 12m or just straight out M (which is the 13th number of the alphabet).
Or its there but can only be accessed by a service elevator etc (conspiracy theories anyone....)
Anyone remember "Max Headroom" ? I do just, he was on the 13th floor...
You'll find that most times its also that the block was split (or sub-divided) - 11 for the original plot and 11a for the new plot.