Why is the cutoff frequency of this highpass filter with formula different from LTspice?
There's no difference in the simulation and calculation, at least none that I can see. Here's what the calculator gives:


The "problem" is that in the calculator the cut-off frequency seems to mean the frequency where there is a pole. And with this given topology at that frequency there is 6 dB of attenuation. You need to find the -3 dB cut-off frequency from the graph.