As for the 18 hole scores, It was a one day glitch where any Coin Room games (1,3 or 9 hole) were counted as 18 hole Ranked Rounds.
So the person probably played a 1 hole or maybe a 3 hole coin game and had a total score of 5, which was counted as an 18 hole RR, therefore the best 18 hole round score of 5.
As for the 9 hole scores, I suspect that they aren’t as low as the 18 hole scores you’re describing and are probably explained by a very good 9 hole round of all par 3’s