My guess is that the SSA didn't factor how poorly American's would treat themselves over the next 65 years. If they had their projections for life expectancy would have dropped inline with what they are today.
Either that or those fantastic flying cars we all though we were going to have by 2011 were supposed to have contributed to a longer lifespan.
If you read in the comments there is some discussion as to why this has happened. It seems that longer lifespan is being somewhat canceled out statistically by the swell of people born during the baby boom and immigration.
Exactly. The US (and most other western countries) are playing the game where you avoid the greying of the population by bringing in a constant supply of new immigrants from poorer countries. This is not, of course, sustainable over the long term.