Apparently some bowlers have an unfair advantage with the ball they have chosen.
There are differences in bowling balls that are not always obvious.
The surface roughness of a ball determines whether it will hook in the oil and also how much direction change there is. Some balls have a cover that has surface spikes that are far apart which creates a lot of ball motion. Plastic balls which have less ball motion have surface spikes that are much closer together.
Effective in April, 2009, manufacturers of bowling balls will be required to use certain specifications for the surace of bowling balls they produce.