Theoretically there is no such thing as parallel.
OK, so when we are not at the theoretical job, but rather the real world job, how do we determine the number of degrees, +/-, off of a point on a straight plane ( the 3.5" wide stud face) that is "parallel" to the plane?
I maintain that, in my photo in this post, that the cable between the staple and the box is getting progressively farther away from the plane of the stud face. The photo represents, arguably, the common "as built" assembly of romex-to-a-wall case on a stud. The cable forms a simple ramp, a wedge, that does not level off and that goes into the opening on the wall case at an angle. Absolutely nothing about the common wall case is going to reliably force the cable to enter the wall case in a plane parallel to 3.5" stud face.
When the cable emerges from under the staple, it is more than 1.25" from the edge of the stud. When the cable crosses that 1.25" boundary it is only forming a ramp of some number of degrees away from the 3.5" wide plane of the stud.
The cable cannot enter the wall case unless the cable deflects away from the plane of the stud.
The cable has to be installed NOT parallel to the stud in order to enter the wall case.
Rob and Don, you state that this cable is "parallel" when it is obviously at an angle of some number of degrees to the plane it is "parallel" to. What document determines the +/- degrees off parallel that is still "parallel" at the "intent" of the CMPs?