Most all stock dimensions are based off the rib. Run a staight edge down the rib. The first measurement is the front of the comb (from the bottom of the straight edge ) The second is the back edge of the comb. and the third is from the bottom of the straight edge to the top of the Pad. In this case the comb is parallel (trap dimensions) A skeet stock will tend to have different dimensions from the front of the comb to the back of the comb. PJH
The 1 5/8" dimensions are the drop at the comb dimensions. With the example you gave above, the gun has what's known as a parallel comb. You would be able to place your cheek anywhere on the comb and keep your eye at the same height relative to the rib, unlike a "field gun" with a comb that slants upward in the front.
On the down side, parallel combs do not allow you to slide your cheek forward or back or to match the distance between your eye and cheek bone. (On a shotgun, the eye serves the same purpose as the rear sight on a rifle. Raise your eye and the pattern will impact higher than it did before the change.)
It is the drop at the comb dimension that determines the the vertical placement of your eye relative to the rib. The correct, individual, stock dimension is controlled by the distance between the eye and the cheek bone, along with the amount of flesh covering the cheek bone and determines what "drop at the comb" dimension shooters' need to give them the vertical point of impact they want.
The greater dimension, 2 1/4", is the stock's "drop at the heel" dimension. What you need for a correct gun fit depends on the length of your neck, the slope of your shoulders and the height of your gun mount. It is an example of the interrelationship of shooting form (gun mount height), stock dimensions and shooter size and shape (shoulder slope).