I am not able to find correct frequency when I increase the sweeping frequency from 50 to 100. Why is that?

Hello, I am providing you the codes I am having issue with. The problem is when I am using the sweeping frequency as 50 Hz, the fft is showing perfect peaks at 62 and 66 kHz. But if I sweep the frequency at 100 Hz, its showing its peak a 61.9992 kHz. I am not sure the reason why? I am providing the code here. Also the image is also provided. You can only run the first for loop and the problem wil appear.