Here is an unexpected one: the rail as a screw mounting hole right in the middle. As a result, if you want the supporting plate to:

1. have a symmetrical set of screw holes,
2. have a screw pattern half as dense as the one for the rail,
3. have plate screw holes right in the middle of rail holes,

...you cannot have a regular pattern. As a result, something has to give in the middle: either you omit some screws, or you add a few more. Conservatively, we went for the latter.

I love these little details, as I'm sure you could already tell...