It has been a while since I looked at this but I believe the app-note equation is how to calculate the best/recommended value for RFB while the equations above are MIN and MAX limits for RFB that the AD5933 can use based on your desired Zmin and Zmax. If you solve all 3 equations and the app-note's recommendation is between the impedance values from the equations above then you are good. If the recommendation is outside the min and max then I think you either need to choose a less than optimal RFB or re-consider the range of impedance you are trying to measure. Also, it's not uncommon to use analog switches to swap in different RFB resistance and calibration impedance.
I'll double check this with Miguel as he's the current Apps expert for the AD5933.