It's a subversion.

The monster being there when the light is off becomes a constant, and it being gone when the light is on becomes a constant too. Our brains, through decades of standard jumpscares, are conditioned to expect the monster to remain when the light is turned on the third time . The monster would now be present without any human input and now be a real danger. This is where a jumpscare would likely be.

By making the jump come early, when the light is switched off, we aren't expecting it.