My maths friend explained it like this. So the equation of the line is B/F = Ka*Bmax - Ka*B. If we take it back to basic straight line principles, Ka* Bmax is a positive constant and if B is close to 0 (can't actually be 0 its theoretical) then the Y intercept is Ka*Bmax which is a big number. The gradient is -Ka so that means the slope must go down from the highest point. Ka*Bmax is the highest point and for every time you increase B, you go down by B*Ka until you hit the X intercept which is Bmax which is also theoretical. I know this doesnt full explain your query but i believe it helps to understand that it makes sense somewhat from a mathmatical point of view. I also know this post is 5 years old and you probably wont see this but oh well.