Interesting, I never noticed that or knew there was a difference between traditions. To me it still just looks like a Dhamma Wheel.
Over at Wikipedia, it is a little vague, but they say "The hub is for discipline, which is the essential core of meditation practice."http://en.wikipedia.org/wiki/Dharmacakra
But then below that they show a Tibetan Gankil which looks like the hub.
We chose that wheel because it looks nice and is a better looking design than most of the other images available that are not copyrighted.
We could see the three symbols as Buddha, Dhamma, Sangha. Problem solved.