I am a music producer and mixer and found a strange glitch in the loudness of tracks on Spotify when listening on my mac Spotify desktop client.
Essentially, tracks are hitting my meters at very different levels - some are hitting more normally for modern music around 0.1 and 0.0, and then others are hitting way below that - the difference is all over the place.
For example, for the same artist 'East Love', their song 'Sweet Arizona' hits at 0.0 and their song Firefly hits at -6.7. Listening to the first three songs of NMF right now, Drake's new single 'God's Plan' hit's at -1.9, the Chainsmokers' 'Sick Boy' -3.8, and J Balvin, Jeon, and Annita's 'Machika' is hitting at -6.0.
This loudness difference is clear to me on both headphones and monitors. I'm routing my computer audio through my Apollo 8 Interface into Logic Pro X and watching the levels.
Most importantly, this loudness difference between songs is not occurring through the EXACT SAME ROUTING via Spotify Web Player!!! I can see everything hitting at 0.0 as you'd expect when listening via web player.
Nor is it apparent when listening on mobile devices, though I can't measure that with meters.