Good point, jacobnev. I wonder how the stats break down, eg: I bet the "retention" stats are quite different for owners who bought hybrids specifically for their better efficiency.
Green Car Reports has a piece up about possible reasons explaining Polk's data:
Study Says Hybrid Owners Don
I liked this one as well: for those concerned with purely financial calculations on whether to get another hybrid (ROI), one major factor against them is the increasing number of non-hybrid vehicles that are offering better and better efficiency.
EG: Just 5 years ago, how many 40+ mpg (highway) vehicles were available in North America? Hardly any, and hybrids/diesels dominated. But look at how many are available now.