Looking at the front of a rail and you see a pair of 45 degree angles on each side of the rail. Point contact rail grabbers attempt to touch (and clamp between) each of these four 45's. If you look at an ARMS throw lever mount, you will see this happening as the levers are closed, but more importantly, you will see a gap between the top of the host rail and the underside of the ARMS mount. So this design absolutely depends on the 45's being perfect and uniform for repeatability.
Line contact uses the much larger cross sectional area of the top of the host rail for contact. This is how Leupold, KAC, and other rail grabbers are designed. In these designs, the opposing "fixed side female groove" engages the underside 45 on the rail and as the opposing nut/clamp is tightened, and this draws the mount down to contact the host rails top surface. The clamp's lower inside edge is now in contact with the underside 45 on its side. This relationship pulls the mount down (using both opposing 45's, towards the side with the clamp, and down into contact with top of the host rail
The simple way to compare these is the difference between a four leg stool (point contact) and one with three legs (line contact). I am sure you have experienced four-leg stools that "woobled", but never a three-leg one.
Unfortunately, the Army went for the point contact approach in the MilStd1913 spec because they were heavily influenced by ARMS mounts.
On the good side, again looking at the mil spec rail from the front, the rail itself sits atop a preety tall "root", this allows clearnances for line type grabber and clamps.
Obviously, both approaches can provide return to zero performance. That being said, my preference is for the three-leg stool, because things in this world just are not perfect, especially is Murphy is lurking about. I like forgiveness features.
ColdBlue sends...