Bigfoot, hello.
I might be wrong about this, but if you're talking about the grass strip I think you are, then the 21 day rule might have something to do with currency at the field in question, rather than in general, as I suspect the instructors and the club got tired of picking bits of plane out various hedges.
This is supposition on my part, but would be in keeping with the (understandably safety conscious) thought processes of those at the club.
Regards
JR