"Dancing is more fun, when everyone dances to the beat"
Modern recorders use complicated techniques to capture the RMS of each and every half-cycle or full cycle. This involves reading various points on the cycle repeatedly (i.e. each and every cycle).
Added to the RMS, Fast-Fourier Transform (FFT) techniques only function if the sampling occurs at the same timed point on each and every cycle. Having the incorrect sampling frequency will throw all harmonic trends into total disarray.
All the above involves making a microprocessor or application-specific integrated circuit (ASIC) do the leg work at a specific speed. Some are clever and manage to detect which fundamental is being used (thankfully there are effectively only two i.e. 50 & 60Hz). Most don't!
I really do not feel like getting involved in any arguments here especially the one that says that all recorders should be able to tell what the mains frequency is. Ok, if designed in the USA the programmer will be inclined to make 60Hz the preference. If the software genius is British he may be more prone to think at 50Hz.
Should the device be employed on the opposite side of the pond to which it was designed, and goes through a power cut, the tendency has been seen to be for the device to switch to the preferred fundamental - and this could render useful data nothing better than waste-bin material.
The Reliable Power Meters' range of recorders has the working frequency right up front together with site details, so it should not be forgotten (but as security can only be set when linking to the recorder for the first time). Other recorders I have seen have this setting so deep rooted in menu structure that it is often forgotten. Be careful!