I believe that the two most common reasons for this are:
1) the ball is already on its second owner. The thumbhole was plugged and redrilled in a different spot to accomodate the new owner's span; and
2) the owner decided that his span was too long or too short, and made a small change, usually 1/4" or less.
The driller can measure your hand and recommend what he considers to be a proper span for you, but if you say that the span feels too short or too long, he's going to change it to accomodate you.
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seadrive
Cogito ergo bowl