I'm not sure why it does this. The odd thing I find is (I've been doing this alot with edges) if you switch into vertex mode and fiddle with vertices a bit, then back into edge mode, then when I extrude an edge again, it works fine.

(It's when I extrude an edge, move it in the world coords (the controller is in the middle of the edges) then extrude again immediately, when I go to world coords it snaps to the object center.)