Shippers pay for priority but also have standard space paid for. If they can get the item closer via a circuitous route utilising space already paid for they will. It may be due to backloading to clear the load from a high traffic area to better use under utilised routes with space available (load sharing).
There is some very fancy new routing software under development that will move items around the globe in very strange, but fast, ways. eg if an item can go from Atlanta USA to Melbourne via 6 points in 8 days, this software will investigate all other routes and may utilise 10 points to deliver in 3 days. It will also review its options at each via node. The maths and computational power involved is very scary.