Why can't pax accept what they're told without assuming they're being lied to? Who would come up with a specific line like that if it wasn't true? get real! What advantage would the airline get from it? Makes no sense at all.
Well first of all it will save the cost of moving the aircraft from Gatwick to Stansted, secondly, there is delay compensation which doesn't apply under certain circumstances, so excuses are made up that will avoid compensation payouts.