The first hunter jump that we'll look at is the vertical. If you look at one head-on, it does appear that everything is on the same vertical plane:
If you look at the same jump from the side, however, you'll notice that the flower boxes in front of the jump actually give it more of a narrow ramped oxer shape. The ground line being so far in front of the jump and built up essentially makes it easier for the horse not to get deep to the jump and not to hit the rail while taking off. A truly vertical jump would require the horse to rock back more and take a rounder trajectory than the longer one encouraged by a ramped jump.
A hunter oxer also looks quite like an ordinary ramped oxer when you look at it head on:
These jumps are designed to be ridden with minimal rider input, which is why they are so useful for the hunter ring. The ascending shapes mean that very little balancing is required in the ring to keep the horse's front end from hitting the rails, which allows the rider to stay quiet and make it look easy.
The great thing about the hunter ring is that since the jumps very rarely vary from this description, you can easily prepare for what you will meet in the show ring. If your horse tends to jump quite flat or long, you might want to practice more square oxers and truly vertical verticals at home because spending all your time jumping ramped fences at shows will only exacerbate the problem.