1. The Naturalization Lag Mechanism
The overwhelming driver of false-positive citizenship flags in voter list maintenance is temporal database desynchronization.
Most states cross-reference motor vehicle administration (MVA) records or old visa databases against voter files. When an immigrant legally residing as a permanent resident obtains or renews a driver's license, the state classifies them as a non-citizen.
When that person later takes the oath of citizenship and legally registers to vote, state motor vehicle records are rarely automatically updated until the citizen next renews their driver's license (often up to 8 years later). Cross-referencing against stale state databases treats fully naturalized Americans as non-citizens.