Most vehicles now have a device in the filler neck that looks like a plastic baseball or wiffle ball without the holes.
The intent was to keep fuel from spilling out the filler neck when making a hard turn with a full tank, or on older cars, out the back if the filler neck was behind the license plate.
It also acts to help shut off the fuel pumps when the tank is full. The gas in the filler tube lifts the ball up and causes a much smaller mount to back up to the neck and click off the pump.
A side benefit was that the ball prevented a siphon tube from being inserted very far into the tank, if at all.
My old 88 Dodge Dakota had this when I had to drop the tank to replace the fuel pump, and so did my 91 S-10.
I'm not sure if GM still employs this method, but anyone who has dropped a tank to replace a fuel pump should be able to answer that.