Hmm, I'd be more inclined to think, if something is triggered indefinitely, it would just keep going when the lever is used, instead of being disabled...
Different train of thought here?
Pressure plates in this game are weight-based, so enough weight on them triggers them
Placing the oil barrel on is the trigger depresses it, enough to set the trap off. You can (and are indeed supposed to) go pull the lever to turn it off. At that point, the trap turns off, even though the barrel is still keeping the plate flat.
Maybe a different train of thought is at work, but something is possibly amiss here. If having weight on them is supposed to make the trap trigger continuously, then pulling the lever shouldn't have shut it off at all. If the plate is already fully depressed, because say, a big honking barrel is on it, stepping on it again shouldn't do anything because there's nowhere for the plate to move to trigger anything.