You say:

"By definition, the chance that you'll die before your projected life expectancy is 50 percent."

Let me "baby" you through this. Say you have three 65-year old women: one of them lives six years, one nine years and one thirty years. Together they have lived 45 years past 65 - their life expectancy at 65 would have been 15 years.

Two of the three lived less than their 15 year life expectancy.

Do you realize that if I add a fourth woman, who lived for 20 years beyond 65, to your example, it puts the number back up to 50%? Or that your example has nothing to do with the sentence you quoted? There are only two outcomes: either we live past our life expectancy, or we don't. That's why it's 50%.