I'd have to look into it in detail (which I won't), but knowing Texas (I lived there), most of it's probably the result of shoddy record-keeping, e.g., a person moving from San Angelo to Dallas, registering properly there, and Tom Greene County never quite getting around to cancelling the expired registration.
Or they could both be valid (unexpired). The usual (though it varies) is that your voter registration remains valid for four years of inactivity. A lot of people move in any given four-year period. Most, possibly all, states do not have a system for deleting an old registration when the voter moves and registers in a new county/district/precinct.
Moosie is warning us of the dangers of the unregulated, dangerous chemical, dihydrogen monoxide!