I remember reading a paper by n00k1e, one of the best q2 jumpers ever, about the differences between jumps at different points compared to the origin, I tried to find it, but I don't remember where it is.
I remember the gist is that the origin works kind of like gravity in this game. So, that means that you have your normal jump height, but then also you are always slightly pulled toward the origin. So, when you are in the -y part of maps, you get extra height, and the opposite happens in +y.
It also affects what fps you need for those really awkward jumps that can only be done at odd fps'.
I'll look for a link sometime soon.