Their suggestion is a better solution, that way you basically hand the dim command off to the local light controller.
From what I remember of the original X10 equipment, I tried the same thing, only in a slightly different manner, but got the same result.
I believe the issue you were experiencing is that when you issue your dim command, the unit turns fully on, then dims down to the level you requested. So your command 'Set Powder Room Light 80%' turns on the light then dims it down to 80%. Then a 2 second wait. The 60% command is then given, which turns the light on fully again, then starts the ramp down to 60%. But before the unit can get to 60%, the 2 second wait after that command times out, then the 40% command is issued. This continues and gives the appearance the light control unit is just changing from bright to 80% until you reach the final OFF command.
Just so you might understand a bit better what might have been happening!
James