backscattered field measurement in CST

Hi all
I define a sphere in CST MWS and a plane wave as excitation.I want to calculate the backscattered field at different position, from nearfield region to farfield. We expect that when we approach to farfield region, the backscattered field converged to a specific value and close to farfield result. but it doesn't happen in simulation!
What do you think about the reason of this?

You forgot to report your results.