[sowbug]

Pretty sure this is a Bayes' Theorem thing, but happy to be wrong and learn something.

I think we both agree the table talks in terms of current age. But making it to age 50 doesn't mean you'll make it to age 51, so you can't add that extra bit of life (the one you think of in terms of a day lived counting a little less than a day against your life expectancy) to the age 50 row in the table. You can't add that bit until you actually are age 51. Otherwise you're counting both a known thing (you did make it to 51) and an unknown thing (whether you will make it to 51 if you're only 50 today) to a predicted thing (when you'll die if you're 50 today). Not exactly double counting, but taking for granted something that's not yet proven.

I believe that extra bit is what the other commenter was wondering about.