In bowling, how come some bowling balls have a small piece drilled off of it?

I’m not talking about the finger holes. In some balls I see at the Bowling alley, there’s a small part of it that’s drilled off, near the finger holes. What is it for?

because the balls are all weighted, and still all the same size. the manufacturer drills a hole into the ball and removes a calculated amount of weight, then fills in the gap with a less dense and therefore lighter material.

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