It doesn't mix up with the rules.
When you jump, technically you have 2 actions, one shooting and one jumping.
Ruleswise you need to ready (first action) and then jump (and get a free attack during jump movement).
Otherwiswe no jumping or flying unit would be able to do any attack.
Why is the difference important? That's really simple.
Imagine there are 10 Bugs in close combat with your MI Troopers. Because of the rules you can't simply jump away - you have to ready (and thus endung up an (ready) action within 10" of that bugs), so the bugs will tear your troopers apart (reaction).
You cannot just jump away.