Unusual Binding curve with streptavidin beads

[DK79]

I have performed a binding experiment using  streptavidin agarose beads and a biotynilated molecule ranging from high to low concentration. The molecule exhibits close to 100% capture at low concentration as is expected then gradually drops off. The drop -off is more extreme at higher concentrations. If the beads become saturated past a certain concentration, should the  drop off be at a steady rate?  Why is there seemingly a slow drop-ff initially then a fast one (it should be noted that the % captured is measured by fluorescence)

Binding curve is below. The beads are Thermo Scientific 7.5% Agarose slurry. 

Thanks!

[CharonY]

It is not at all unusual, the laws of mass actions predict non-linearity. Read up on "fractional occupancy" in the context of Clark's theory.

[DK79]

i will read up on it. I know i have seem this type of curve in the past but have not been able to explain its nature.

Thank you!!