While it would be difficult to verify the exact number, the claim is plausible.
De-registration of voters when they are no longer eligible to vote is not something that happens efficiently, for a few reasons:
- It is a difficult, time-consuming administrative task. Counties don't have good information on who is resident at any given time.
- There is no real incentive for counties to de-register people.
- Purging voter rolls raises fears of disenfranchisement, so there are often safeguards making it hard to remove voters.
Take the policy for Florida as an example:
Under Florida law, a registered voter may only be removed from the
rolls if he or she requests in writing to be removed, if a Supervisor
of Election received notice from another state election official that
the voter has registered out-of-state, if the voter is ineligible, or
if the voter fails to respond to an address confirmation final and
there is no voting or voter registration record activity for two
subsequent general election cycles.
And even this only says the voter may be removed, not must be removed.
This page also gives some numbers for Florida: over the last 21 years, 15 million voters have been registered, but only 7.2 million have been de-registered. While the population of Florida has increased, the 7.8 million additional registered voters are several million more than the state's population growth over the same period.
There are several programs aiming to compare registrations between states and clean up voter rolls. Supposedly one of these efforts found "five million questionable registrations" (though it isn't clear exactly what this means). The mere existence of these programs suggests the states believe this is a huge problem--and none of the programs covers more than half of the states, so they will only be able to identify a subset of all duplicate registrations.
In short, it is a virtual certainty that nowhere near 100% of ineligible voters are removed from voter rolls in a timely manner. And this makes it plausible that some counties may end up with more registered voters than actual residents.