[throwaway0665]

Does this take into account capo position? A G is easy to play so authors might use G to play a Bb for example with a capo to avoid barre chords. Likewise authors will choose simpler chord substitutes to make it easier to play.

It's the same with lead sheets / the real book style music books. Performing musicians need to reproduce music quickly so only the triad will be written down even if the musician ends up playing some other extensions.

The data is heavily biased towards simplicity. You can make conclusions about the data - but not music as a whole.