Even with simple plastic balls, there are several factors that play into how much one ball will hook when compared to another similar ball.
Being plastic, and having simple pancake weightblocks, shell hardness, surface finish, and static weights will have quite an influence on them, not to mention the difference in each bowlers delivery.
Is one of those balls more aggressive than the other? Even though they are designed to be quite similar, odds are that one of them has more of the proper characteristics that will allow it to hook than the other one does.
In the old days (when the pros were throwing plastic for strikes), Many players used to illegaly soak their plastic balls in different chemical compounds to soften the shell and make the ball hook and hit harder.
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