I am currently a 14 on both of my home courses, I regularly shoot 39's for 9 holes. That does not mean I have 39 on the back, but I think 9 holes is easier to score, than keeping a round going for 18. Today I shot an 88, but lost 7 strokes in the last 4 holes. Some of us are streaky, some people shoot the same front, back, almost every round. Does having the ability to shoot a 39 one every 4-5 rounds make me a sandbagger?
There are two main types of sandbaggers, the cheating kind, and the cheating themselves kind. I tend to think there are more reverse-sandbaggers than the high posting type. I see people giving themselves 6-3 footers all round and then wondering why they didn't do so well in a tournament where they had to grind those putts out.
There are also unconscious incompetent sandbaggers who just plain don't know the rules, and score wrong. You see this more on public tracks than private, however I have been shocked to see people break rules in total ignorance of them at my own club.