"Being declared 'inactive' would then start the clock ticking on the years-long process that culminates in being cut from registration rolls, although an 'inactive' voter in Georgia is still legally registered to vote and by law has full access to a ballot."
So "inactive" people can still vote. You are only denied the vote when you are cut from the rolls.
From the 591k....I would also make a bet that at least 10,000 passed on within the last three years, and people screwed up not removing as they were supposed to. Some are likely military personnel who transferred to another state, and signed up there to vote.