Since its phase in roughly 1998~2000, ORVR (Onboard Refueling Vapor Recovery) has been required by the EPA. Instead of belching a lot of unburned hydrocarbons from the filler neck during refueling events, this system vents most of those vapors to a new, larger carbon canister where they are captured and stored for burning later. If the vapor path from the tank to the canister to atmosphere is restricted, the tank will be difficult to refuel because the traditional fuel filler vent is smaller than it was before ORVR was introduced.
There is a step-by-step troubleshooting procedure in the factory shop manual for poor filling.
Here's a generic diagram of the Ford ORVR system: