Why does my cutoff frequency not match the 3db down point?
Because that is a 2nd order filter &amp; as such the cutoff frequency is:

\$\frac{1}{2\pi \sqrt{R_1 C_1 R_2 C_2}}\$  and the rolloff at this frequency  is dependant on the damping factor 

https://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology#Application:_high-pass_filter