We are assured that only a small number of legitimate votes are canceled by vacant lots, dead people, trees, pets, illegals, vacationing New Yorkers (in Florida), and extra-dimensional addresses occupying the null-space of neighborhood numbering schemes.
How they got there and who they consistently vote for is unimportant - only that the problem is small enough to ignore.
My mother was still on the voting rolls when I voted in 2008 despite being dead for 7 1/2 years at the time.