Different technique is all.
A boxers hook is with your thumbs vertical upon impact, as you said but they also keep the rest of the arm straight in line with the fist. I think this has a greater ko potential since you're able to throw more weight into the other guys face forcing the movement that hopfully rattles his brain.
A MT hook is with your palms parallel to the floor as well as striking with a bit of a downward motion to keep the elbow above the fist, unlike the boxers hook this punch can be turned into an elbow quite easily if needed or desired.
Last edited by Hibernaculum; 01-10-2013 at 12:26 AM.