The ECM uses different methods of verifying the fuel level in the tank to help with inaccurate readings(fuel sloshing in the tank) I actually think there is four different methods.
When the key is off and then turned on it only take a few seconds to move the gauge because in the 1-2 seconds after the key is turned on its reading the sending unit and if the ohms don't change it instantly gives your gauge a reading.
Now, key off (while fueling) and then the key is turned on and in that 1-2 seconds (as the ECM is reading the sending unit) the ohms are changing and the computer thinks the fuel is sloshing around and so it defaults into a mode were it changes the gauge takes up to 60 seconds to go from empty to full(anti-slosh mode). This mode always being used while driving to prevent the gauge from bouncing all over the place like all the old cars and trucks do.
The mode I believe you experienced happens after the ECM defaults into anti-slosh mode and fuel is rising rapidly. I believe its called recovery mode. Its where the ECM is in Anti-slosh mode and (for example) the gauge is reading 1/2 tank and the next series of readings keeping showing the gauge at 3/4 and rising. (gauge reading is below actual sending unit reading) This makes the computer go into a so-called limp mode and cause the ECM to take up to 20 minutes from empty to full. I believe this mode is used to prevent the gauge needle from jumping up in increments every minute or so.
The times for each mode might be off slightly...
As for dowmace, I believe his gauge read normally because he doesn't shut the truck off causing the ECM to take up to 60 seconds to reach full but filling the tank takes longer so he never see the problem. This problem only occurs once the truck is shut off and them turned back on while fueling... Hope that makes sense