I just don't get it ...

I went into our Beijing office today for a meeting, to grab a teacher's manual, and to pick up my passport and visa (I am now entitled to stay here for a year, YAY!).  I was in the elevator, and noticed that the building does not have a 13th or 14th floor, but 12, 12a, 12B, 15, 16 etc.  I inquired as to why this is, and asked if it was common here.  In China, numbers are pretty important.  Some numbers are good luck (8), while others are bad.  This is evidenced by the cost of phone numbers.  Many 4s in a phone number will be cheaper than a mobile number with many 8s (highly sought after).  I am not sure why 8 is so popular, but 4 in Mandarin is "si", which sounds almost the same as "die", ("si" but with an accent on the "i"); hence the bad luck.  Also ... 13 is unlucky in China just like it is unlucky in most other places, hence the absence of the 13th floor.  There's no 14th floor because 4 is an unlucky number.  I have to wonder: if this is true, WHY is there a 4th floor????  If 4 is an unlucky number, why is there a 4th floor, but no 14th floor? For the record, a Chinese person explained this to me.  When I asked him why there's a 4th floor, he just didn't know.