i'd actually love to hear someone who has done hunter derbies talk more about how they are judged/scored as my focus is jumpers, eventers, and fox hunting and i'd love to learn more there myself!
I met some foxhunters who invited me to come ride with them and I never got a chance. Now I really regret it! I think I'm going to have to do a little more riding out in the field! Talk about addiciting!
Derbies are judged off two rounds. The first round is judged like a typical hunter course. Stress placed on distances, lead changes, strides in a line, consistency, and as you get more competitive quality of jump and movement. I haven't done a lot of dressage shows but the ones I did it seemed that you could make a not huge mistake and still get a decent score if the rest of the ride was super. The hunter ring on the other hand, if you make one mistake you can be out of the ribbons. It doesn't matter how great the 9 jumps are, if you biff to the 10th. As you mentioned earlier, there are usually at least 2 or 3 judges for the derby and after each round your scores are announced from each judge. The scores are combined and the top 12 are called back for a handy round.
Each round is based out of 100 points (this is every hunter round). I have NEVER seen a 100. The only rounds I"ve seen in the 90's (which are VERY rare) are spectacular rounds at the top divisions at top shows, like the Working Hunter at Indoors. A trip in the 80's would be correct distances, correct lead changes. There might be a very slight bobble somewhere but altogether a very nice course. Often something in the low 80's can win in a Classic around here. A score in the 70's is an ok round. Maybe there was a late change. Maybe a distance was a hair off. A score in the 60's means there was a horrible distance, maybe several bad distances, maybe a missed lead change or two, a hard rub, or usually some combination of the above. A score in the 50's is usually an accidental trot, a knocked rail. A stop will get you maybe a 40 if the rest of it's decent. Now, for the handy round "handiness" is weighted quite a bit. You can have a great course, but if you dont' take any inside turns your score won't be very good either. You also can increase your score by taking the more challenging options.
So the last derby I did, I missed a change and got I think like a 72 and 74 but got 4 extra points from each judge for doing the options. In the handy round I didn't make any mistakes but I just didn't ride the course "handy" enough (didn't take many inside turns, started my trot for the trot fence way early, etc) and so got like a 76 plus 4 points for the options. I'm still kicking myself for not being a bit more daring because the comp wasn't great and I think I could have won it with a better round. In another derby I did, I had a God awful biff (oh man was it terrible) and I think a late change but I took all the options (and there were a lot). So my score was a 62 and 64 but I added like 6 extra points to that. Still didn't make it to the handy round, and with that biff I probably shouldn't have. ;)
That's a bit of a novel. But does that tell you more about scoring??