I agree. It's far too short of a time span to bring evolution into the picture. Improvements in medicine and sanitation, as well as improved diets are responsible.
I wouldn't think it would be too surprising to see an ever increasing life-span, however, as our understanding of the human genome continues to progress and we develop the technology to manipulate it and fix the errors. Wish they'd hurry up with the research, though. I ain't getting any younger.