Washer "wall outlet must not be located behind dryer."
Some of those are up/down washer/dryer combos, and the dryer is actually mounted to the wall in a semi-permanent fashion. As such, you can't have the washer plugged in behind the dryer because it is no longer in a freely accessible receptacle, electrically the washer is sort of permanently installed itself (which is a bad idea in general.)  The ones that aren't, I suspect, are guilty of a lazy GE washer manual writer.  It is not a code requirement of any sort.

*edited with more detail