I believe the direction of the way the meat is ground is what makes the difference.
Meat will expand in one direction (I forget which, either with the line of extrusion out of the grinder, or else crossways to it.)
This is the principal behind those “plump when you cook’em” hot dogs- They are formed crosswise to the standard arrangement, so instead of shrinking diametrically, they shrink end-to-end and get plump and short.
If you have a fresh batch of meat, try to form one patty with the grain and one against it, and see if they aren’t different. (I’d do it now, but I’m out of hamburger today.)