Unusual Binding curve with streptavidin magnetic beads

[DK79]

I have performed a binding experiment using a biotynilated molecule ranging from high to low concentration. The molecule exhibits 100% binding at low concentration (as expected) then gradually drops off. The drop -off is more extreme at higher concentrations. If the beads become saturated past a certain concentration shouldnt the curve drop off quicker and at a steady rate? (it should be noted that the % captured is measured by fluorescence)

Binding curve is below. The beads are GE's Sepharose Mag beads  cat # 28-9857-38

Thanks!