Losses in NE555 oscillator and Sallen-Key filter
@Snickers,
You can blame ;) it on Laplace who invented the formula for time to frequency domain transformation and Fourier for then deriving the frequency components. 



I assume you are using the same formula fo S-K BPF?


H is gain and Q is shape factor fcenter/deltaF (-3dB)
 There are better filter designs and better sine wave designs too.



Here two matched Rf's control center F and one Rq controls Q.