It's just a consequence of the ridiculous party affiliation declaration of voter registration in the USA. It may or may not have anything at all to do with how a person actually votes, and is not a record of how people have ever voted (except coincidentally, if they both registered, say, as a Republican and voted for all Republican candidates). Elsewhere, party membership/affiliation is completely separate from voter registration; participation in the candidate selection process, etc., is governed internally by the party and its membership.