Atrios points out the obvious, in response to “there are more voters on the rolls than people in town!” cries:
When people move, most do not bother to contact their local elections board and ask to be removed from the voter rolls. When people die, most do not bother to contact their local elections board and ask to be removed from the voter rolls.
I was probably part of this problem when I was in college. I suspect there was a time in Georgia in which I was listed on the voter rolls of at least three different counties in the Atlanta area. Whenever I moved I just registered in the new county. It never even occured to me that that action didn’t simultaenously remove me from the rolls of the previous county. I’d happily support a plan to put such a system in place, but there are about a dozen more important elections issues that should addressed first.