On Henry Lewis Gates's TV show, "Finding Your Roots," he referred to a guest's Russian Jewish ancestors as "ashkenazim." didn't Gates err?

Knowing nothing abut the particulars I suspect there is considerable overlap, and that the categories are not mutually exclusive. That is, some Russian Jews are not Ashkenazi, and some Ashkenazi are not Russian, although they are indeed Jews.

Hab knows more about this than I do, but Wikipedia (I know, not the best source) does show that that in the Middle ages Ashkenazi Jews, due to persecution, fled to many places, including Russia.