If your the only one on the left, the changes happen. they just don't happen as fast or to the extent that they do on the right.
As you say you notice changes after a "few" games. On say a league with 5 man teams, you can notice changes on the right side after just 10 or 15 mins of warm up.
You see 1 bowler during warm up could bowl what amounts to almost a game. So if you have 10 bowlers (9 rightie's & 1 lefty on a pair) the 9 rightys could bowl what amounts to several games on the right( depend on how fast they go). while the lefty would only have bowled around 1 game on the left and all this just during warm up.
So to a extent there's something to it.
So does the left have a advantage? Well sometimes, sometimes not. The left not breaking down as fast on some conditions could help you. But on some conditions you might wish the shot did break down faster.
As for keeping track of righty and lefty's on leagues, yeah they do.
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