I find the rolling under makes sense.
d20 : as you increase a skill your bonus to your rolls increases, making it easier to score a success based on a static set of DC's. The DC for jumping over that low wall never changes, you just get better at doing it. Its a fair enough system.
GURPS : as your skill increases you increase the target number required for success, therefore making it easier to roll a success. To take into account attempting more difficult activities you can either (and I dont know which GURPS does) apply penalties to the target number, by lowering the target score, or apply penalties to the dice roll, by increasing what is on dice.
Both systems work just as well, and its only a matter of personal preference which you would rather use and which you can get your head around easier. For some people its easier to work with the static DC's, while for others its easier to work with the fluid success targets.