ADIsimPE Sallen key low pass filter Bode plot

I was trying to simulate the Bode plot for a Sallen Key low pass filter (R= 10k , C= 1nF) . The filter was designed for a cut off frequency of 15.9 kHz.  The simulated filter response however shows a 3db frequency of 10 kHz .

Could any one help explain why the simulated frequency response is not as expected and what I am likely doing wrong ?.

I have attached the schematic and the response obtained.