Yeah, and those are the people who probably aren't voting. In theory (from what I interpet your post to mean), do you honestly think that a bunch of illegal migrant workers will risk deportation and their livelihood over casting a ballot in Arlington county? Probably not. This is a place where the average resident agonizes over fair trade coffee, organic food, sustainable "green" living, and Apple computers. They and all their friends will all run out to vote on election day.
What I mean is that the registration figure as a percent of total population is too high because, as you say, the illegals are not voting (assuming they were counted in the census in any way). The heavily minority makeup of the schools makes me believe that someone is counting them. I have to put up with other affluent Arlington pre-school mothers every day. Surprisingly about 40% are closet Republicans. The non-child bearing ugly single policy wonks out number the real americans 3 to 1 but still can’t make up 98% of the registered voters.