There are far too many differences between pot games and league play for you to be considered a sandbagger for doing better in pot games. Just to name a few:

-The play in pot games is much quicker which allows you to maintain a good tempo.

-There is none of the BS happening between shots like in league arguments, card                         games, tons of time to stew about last shot, etc. There is trash talking, but it's great to have so much fun while bowling and win some money on the spot.

-Massive amounts of alcohol consumed, most people I know don't drink any if at all during pot games. A little different on league night.

-As mentioned the amount of focus paid to your game and each shot is ten fold compared to league play. This allows you to remain in the zone for the whole game not in and out of it like in league.

I for one can only hope to bowl as well during league as I do in pot games, tournaments, and pro-ams. Just a total different atmosphere.
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"A little nonsense now and then is relished by the wisest men."
-Gene Wilder