Core shape doesn't really matter, that's just marketing, it's where the mass or weight is placed. They could change the core shape on every ball if they wanted to. Part of it is building familiarity, if you like a ball, and see it's got the same core, but maybe with a different coverstock, you probably will buy it to fit another place in your arsenal. And like somebody already said, cover accounts for the majority of ball reaction. Take a plastic ball and sand it to 200 and see what happens.
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This is Fluffy. He is the Destroyer of Worlds.