I'd be happy to!
If you can picture it, both resorts are on the right side of the same dead-end road. If the bus passes Poly and stops at GF first, it would then approach Poly with Poly on the left side of the road. For whatever reason, the left turn into the Poly complex is very tight, too tight for a full-size motorcoach to make the turn in one swoop ... it would have to enter using a K-turn (you know, turn in, then back up and cut the wheel the opposite way, then go forward again), and you can imagine how dangerous and disruptive that would be.
So this means that ANY full-size bus, be it DME or Disney Transport, absolutely must go to Poly first if it is also going to stop at GF.