how can the frequency response of Matlab plot differ from the calculated one?
If I measure the cutoff using two other methods, I get about 38kHz. 


Frequency when phase = 90 deg.
Extend -40 dB/decade line until it crosses 0.


Conclusion: Not a Butterworth. A schematic would help us further diagnose it.



Calculate zeta and Q from your parameters:



You can use an online filter calculator to set Q, or the wikipedia article has some nice equations. If you set m = 1, and n = 1.414, you will get a butterworth (Q = 0.707). The caps will have a ratio of 2:1, and the resistors will be equal. This is a common design choice, but there are others.



https://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology