The first judge only saw one cow slip out the gate, while the other judge saw 3.
Seriously, it's a judged event. It's that person's opinion of your performance and that's all it is. It wasn't just taking your hand off the gate, it's the quality of the moves that go into opening the gate, it's how far you open (or really don't open) the gate, the amount of time it takes to do the obstacle, it's the sidepass to the gate... It's 7 out of 10 points. That's still a solid score but there is room to improve. If you'd been given two 9s would you be questioning that performance of the obstacle?
I would much rather have a judge that was willing and able to point out my lapses and give me stuff to work on at home than to go into a new event and be led to believe that I already know what I need to know and do.
Taking your hand off the gate is huge in my book too. Most people don't do a good gate and that one obstacle won me many classes as a kid. A horse that walks off when I'm on the ground? It's a long walk home and I'm guessing that judge knows it.