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Your Average score will not continue to make 'large jumps' as you play more games. The more games you play overall, the longer it will take for your average score to get better. This is normal with any scoring system that has averages.
If you have 500, or 1000 games played - you will have to bowl MORE games better than your average, in order to make it rise. Having 15 games that are only a 'few' better than your average - may not make it change right away. Someone with less games, such as 15 or 20 - will still change more dramatically as their games played grows and their Average levels out.
after going over your old 15 times lol