Sorry if the wording was a little confusing, what i am saying is at times due to the mechanical shifting there can be abit of a lag between the motor getting the signal to disengage 4x4 and the actual motor disengaging, take for example you are driving through a wet field using 4x4 you get to a gravely road or a tarmac road with plenty of grip you shift to 2wd sometimes the system will not disengage the front wheels straight away as there is pressure on the gears caused by slight wind up, you sometimes have to snake a little etc to get it to drop back into 4x4, if you carry on regardless the chances are the system gets more and more wound up and won't drop out, not saying this is the case every time but it does happen.
Now if you are in auto you are completely unaware of when the system is in 2wd or 4wd, not sure if the dash flashes up to tell you what's going on but your unlikely to notice it, so if the system tries to disengage but the mechanical gears don't you could be driving on tarmac with the system still in 4x4 due to the wind up locking the gears together, sort of like trying to pull the truck out of gear under load if you judge it right you can change gear without using the clutch if you don't you probably won't be able to get the box out of gear due to the pressure on the dogs