Realistically, the course ratings don't factor in alot of stuff that can affect an individual player.
For example, 2 courses may have the exact same number of hazards, the one a guy plays all the time are 75% on the right side of the fairways, and the tournament course has them located 90% on the left side of the fairways. If this guy's misses are regularly to the right, he'll end up in alot more hazards at his home course (resulting in a higher handicap) than during the tourney.
Wind can be another factor that isn't taken into account by slope - playing on a protected course regularly then getting a tourney on a more exposed course with the wind blowing.
I obviously don't know whether these folks are sandbagging or not, but I don't find it odd when a 10 handicap shoots anything between Even Par and 100.