The point is that you chose to go to the loo therefore you weren't there, therefore they could push back and depart a bit early. You have your cause and effect the wrong way round.

If you had just gone straight to the plane, and arrived at the gate at 7.14, they would have processed you and got you onboard even if that took longer. They would not have left early.

As it happened, you went to the loo instead. T-20, you were not there, gate closed without you. It seems everyone else was already onboard and settled, and the remaining pre-departure tasks did not take long and so they were able to get away early because as far as they were concerned everybody who was coming was already on and so they could grab the opportunity for a runway slot at 7.19.

But they wouldn't have gone early if you'd arrived at the gate at 7.14, been allowed through and were still ambling down the jet bridge or taking your seat at 7.19, as they wouldn't have been ready. Until you chose to go to the loo, there WAS no early departure.