Not really sure about the biomechanics of it jahmez. But I believe there is a good reason to usually finish off with the left hook, and especially if you've just scored with your 1-2. Any of those first 2 shots if scored has likely caused the person to by instinct bring themselves back to guard and naturally expect another down the middle...so the left hook is a great way to finish and can get around the guard. Plus it just feels more natural throwing at the end of a combo.

I know what you are talking about tho. I find when throwing 1-2-1 it seems the force is being taken away by the finishing jab.