Actually, my understanding of how it works is the drop system works like 2 roulette wheels: a primary wheel to determine a non-rare drop or if you get to have a chance at a rare one, and a second one that determines what valuable drop you get if you get lucky enough to activate it with the first wheel.

Now from what I read, the RoW only has the above illustrated marble effect on the second wheel, and doesn't activate for the first one.

As for where I read this, I think it was from a J-mod on the official forums, but it was quite a while back so I can't be sure TBH.