2. Yes, it's not an uncommon thing. People move. And how many of those 160 counties has a university/college? (Answer: probably lots. There are about 4100 colleges and universities in the USA) Student comes in, is there for 1, maybe 2 elections, then goes somewhere else, and doesn't bother to inform anyone they're leaving the district (who does?). And they probably didn't inform anyone when they moved from home, so they might be registered in 3 places and because you've got a seemingly random distribution of responsibilities between levels of government (whoever thought having state governments managing federal elections didn't plan on your country getting this big), you get massive left-hand-not-talking-to-the-right-hand situation, resulting in the rolls getting cluttered, but having to err on the side of caution so you don't mispurge someone.
We have less of a problem, as our elections are done federally by Elections Canada, who can get their info from the CRA and also statscan.