If you'll look at the stats you will find many balls with similar covers and the same RG & differential.

Sometimes, especially with coverstocks, companies are intentionally copying each other.  For example, when particle coverstocks first came out almost every company followed suit.

But in many cases, the ball makers are simply coming to the same conclusions.  Company A may use an old core and add flip blocks while Company B makes a one piece core to get the same stats. There's only so much you can do (i.e. dry lanes need milder covers and higher RG).

The ball companies are in tight competition with each other and they've learned lessons from past mistakes.  They don't want to ruin there company by not keeping up with their opponents.  (e.g. Faball never fully rebounded from sticking with urethane too long.)

You will also find the opposite.
Pictures of cores will look identical yet their RG and RG Differential values will vary wildly because they use different densitiies in the pieces that make up the core.

Conclusion: can't tell a book by its cover (or picture).

Bob, I disagree with that. You can have similar RG and Diff numbers, but one core is symmetrical and the other assymetrical. The 2 balls will roll different, due to the difference in core shape. Once the cores are put into motion, dynamics take over.

Dynamics take into consideration a 360 degree rotational plane of the core, whereas RG and Diff numbers are only measured from 2-3 specific points or planes. =:^D