Here’s the totally not scientific answer (although there is some sort of science behind it). For most parts, those cadences are synced to the music BPM as a 1:1 or 1:2 factor.
I say most because I couldn’t get it to work with all the tunes.
So when you have a 72RPM (and like James says, why not after all. We don’t always pedal to a multiple of 5 when we ride outside, do we?), it is because the tune is at 144RPM. An 82 or 84 RPM will be a tune at 82 or 84 BPM, etc…
This is a (very loose I’d like to add) attempt to interpret research the “Effects of Music on Cycling” by Pr Costas I Karageorghis.
But really, it is also because no one pedals at super high cadence on cobbles. So that wouldn’t be very good to have you do it, would it? 
As you probably know, the key to “smoothering” the cobbles is to pull a low gear and churn it. Glad you enjoyed the workout!
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