In other words, the registration rates of those counties exceeded 100% of eligible voters. The study found eight states showing state-wide registration rates exceeding 100%: Alaska, Colorado, Maine, Maryland, Michigan, New Jersey, Rhode Island, and Vermont.
The September 2020 study collected the most recent registration data posted online by the states themselves. This data was then compared to the Census Bureau's most recent five-year population estimates, gathered by the American Community Survey (ACS) from 2014 through 2018. ACS surveys are sent to 3.5 million addresses each month, and its five-year estimates are considered to be the most reliable estimates outside of the decennial census.
Judicial Watch's latest study is necessarily limited to 37 states that post regular updates to their registration data. Certain state voter registration lists may also be even larger than reported, because they may have excluded "inactive voters" from their data. Inactive voters, who may have moved elsewhere, are still registered voters and may show up and vote on election day and/or request mail-in ballots.