Hi Gary
While Matthews explanation is a bit off, the solution is correct. You do have three origins: Project Origin, User Origin and Edit Origin. When you change your coordinates to be absolute instead of relative (by clicking on the delta signs in the coordinate box), ArchiCAD is now referencing everything relative to your User Origin (which may be in the same position as your Project Origin if you haven't relocated it).
This is obviously quite logical, and is also why your guidelines also reference your Project Origin. If you hold shift down, you'll notice that the Edit Origin does in fact jump back to the beginning point of your line as expected. This is because the angle is now constrained relative to the start of your line, so the Edit Origin has little choice but to relocate itself there.
The best solution for you is to reurn your Coordinates to Relative again. Hope that's clearer.
Cheers,
Link.