It's that "infield single". Presumably the scorer thought that while not technically an error, it was someone's mistake, and the run shouldn't be earned. A slow roller where the fielder threw, late, to 1b rather than home?
I've seen plays scored as "fielder's choice" when everyone was safe. That makes no sense to me - not an error, but not a hit, either. The scorer is saying, "he should have been out." But then if a run scores, it's earned. ???