That is a very good question! I have absolutely no idea It would probably have to do with were the egg was? I'm sure that someone will know !

http://www.indianchild.com/Pregnancy/human _fertilization.htm

The above link has all the info you will need. Hope it helps .

Rene - it can take anywhere from about 3 days up to 6 days (that's pushing it however) for the sperm to reach to egg. This is why some women will get "implantation bleeding" about a week after ovulating. I never did, but some women can!!!!
Typically, a sperm only lives about 48-72 hours. It can be longer, but the environment has to be perfect!!!!