Talked to a nurse yesterday who used to live in Illinois. She took her 84 year old grandmother to vote in the 2020 general election. When she got there they said she had already voted, which she had not. The granddaughter said she was told this is pretty common, assuming the older people will not show up to vote.
That is likely what happened here....
Someone thought that the relative wasn’t likely going to be voting, and changed the registration so the vote could be banked and used at some point.