We do not really know why. It's like trying to figure out why somebody won the lotto. Statistically speaking, somebody will. Same with longevity, statistically speaking, a few will make it pass 100.

Edit: I found this about Jeanne Louise Calment from Arles, France who lived to 122:

>>An interesting side question is how Jeanne Louise Calment managed to achieve such an advanced age? She smoked until the age of 117 and reportedly ate a couple of pounds of chocolate a week. She also rode her bike until the age of 100 and lived on her own until 110. Go figure.<<

You'd think the smoking and sugar from all that chocolate would cancel out the benefits of riding her bicycle.

[1]http://www.genealogyintime.com/GenealogyResources/Articles/h...