I agree with all of the above. It's all based on how many people are expected to be heading to which resorts at what times. Someone going to Contemporary today at 5:00 p.m. might get off first from a bus that is also taking people to Wilderness Lodge and Polynesian. But at 8:00 p.m. today, people heading to Contemporary might get of last, after stops at Pop Century and Grand Floridian.
They do have the resorts grouped into zones (Magic Kingdom, Epcot, Animal Kingdom and DTD), but those zones still aren't bus routes. If a bus has guests all from one zone, there can be up to 4 resorts on the same bus. On a bus combining resorts from 2 or more zones, there is a 3 resort maximum. So as you can see, there's no requirement that resorts from the same zone must be on the same bus together. They group the resorts on each bus based not only on geography, but on reservations/demand, as well. In other words, if at any given time the only MK-zone passengers with DME reservations are going to Contemporary, it won't be a bus of only COntemporary guests (unless there are enough of them to nearly fill the bus) ... they'd put the Contemporary folks on a bus with folks going to 2 other resorts, resorts possibly in one other zone or possibly in 2 other zones.
And the resorts aren't necessarily in the zones you'd think they'd be in. For example, most of us think of Fort Wilderness as being in the neighborhood of Magic Kingdom (after all, it's just across Bay Lake!), but FW is actually a Downtown Disney-area resort as far as DME goes, along with SSR, OKW, and the 2 Port Orleans resorts.