For shipments from Europe to USA, it can also matter where in the US that the shipment arrives - I've noticed that packages coming through NY City from Royal Mail to the US Post Officve can spend a week or two in Customs, where packages shipped via Fedex, DHL, etc go through their own customs clearance center elsewhere and get through the customs step in hours.
Several years ago I had a package coming across country from California to New York by UPS ground service. The tracking showed it working its way east state by state halfway across the country to Indiana, then it turned around and went southwest again, all the way back to New Mexico. I thought the package had hijacked the truck and was heading for the beach in Baja, but the other packages must have gotten control of the truck again and it spent the next week coming back again!

Have had other packages from Boston go back and forth between the same depots in Massachusetts and New Hampshire a few times before getting on the right truck...