It's not MP3, it's Ogg vorbis

They're encoding the top songs with higher resolution, the next bunch ( it's still millions of songs) with about half that bitrate and not planning to save the next tranche which is like a looooot of songs but each has less than 1000 plays over the history of Spotify.

This is what I vaguely remember after reading the whole original post AND after a night drinking.

No nitpicking.