I can see where some guys that have the capacity to post some very low scores but don't during every round because it's at the detriment of their enjoyment might appear to sandbag. Their actual intent isn't to steal trophies or money in competition, it's just to enjoy their casual rounds of golf. In this type of situation it makes the European method for managing handicap a much better method as it eliminates all casual rounds from being factored into the handicap.
If I want to play my best, my head has to be in the round and I have to be focused. No one wants to play casual golf with Mr. Intensity so I try to loosen up, have a few laughs, not read every putt or manage every shot perfectly so we can keep the round moving and have a good time. If I'm playing in a tournament I'm going to be more serious, I don't like to lose, I will give 100% effort and I will sometimes post lower scores as a result of my increased focus and attention to what I'm doing on the course.