I don't get it, and it does change some as most signs made to change. I will say at DHS & Epcot, POP (which we stay often) is usually the last one ... but at MK we always chuckle that CBR almost always is after us (and AoA). AK seems like their signs are more permanent but with the new depot area there imagine it changes.
Here is a MK example which seems common to our trips.
http://**************.net/wp-content/uploads/2014/01/Mgaic-Kingdom-Buses-from-**************.net_.jpg