for i increasing in increments of 50 until the API request returns nothing.
There are items which don't appear in the list, but which are Spotify-owned playlists. For example the playlist with ID 37i9dQZF1DX0QKpU3cGsyb does not appear in the returned JSON from the iterative process above, but is authored by Spotify and its details can be accessed via the API directly.
Yes, I've noticed this as well. Another playlist that doesn't appear as part of the API results is 37i9dQZF1DWYa1w6FMMTnG
You can also observe this behavior by going to the Spotify user page and scrolling down until all playlists have loaded -- the playlist noted above, along with the one mpd posted, will be missing from that page.
It seems there are a number of official Spotify playlists that are not actually associated with the Spotify User Id