I have a BX2200. Just to be clear, you are setting the height knob to what you want (the knob near the floor on your left hand side) and the deck just drops all the way down?
Does it stop here you want it and then gradually leak down, or does it go all the way down with the lever regardless of what height adjustment you have set?
I have never really looked, but by the way mine acts I would think that the height adjustment knob adjust the height by providing a mechanical stop for the deck arms.
Same principle as when people put a chain on the 3pt arms to maintain a consistent height.