@ShortcutToMoncton Because the height of the heel bail(metal part) is 53mm, and MS's heel height is 50mm or so. Thus when you step into the bindings, it would be the heel bail holding all your weight, your sole won't be touching the bindings' heel surface, like the drawing below.(yellow part is the boot's heel, and there would be a gap if you use f2 race titanium with MS)
Today I found on the description page of MS's own version of f2 bindings(named Mountain Slope f2 bindings), they say they lower heel bail by 3.8mm, so the situation above won't happen.