Songs on Deezer Desktop sound much louder than on Spotify Desktop

I recently switched from Spotify to Deezer (I absolutely love Flow and the genre/mood recommendations — I’m discovering a lot of new music that Spotify never really recommended to me), but I noticed something unusual: tracks in Deezer on PC sound almost twice as loud as on Spotify at the exact same volume level on my wireless headset. Spotify volume normalization is completely disabled.

What’s interesting is that if I enable volume normalization in Deezer, the loudness becomes almost identical to Spotify (with normalization still disabled there). Why does this happen? I found another thread discussing Deezer’s loudness normalization, but not in this specific context.

My assumption is that Deezer streams the audio signal much closer to the original mastered output in FLAC without additional processing from the platform (even if I switch from HiFi to 320 kbps, the loudness does not change at all), exactly as intended by the sound engineers, so some tracks may naturally sound louder or quieter than others. However, they clearly do not seem to follow the same -15 LUFS normalization target that Spotify appears to apply even when normalization is turned off.

Another possibility is that Deezer’s desktop app simply outputs a hotter signal or uses some kind of built-in preamp/gain compared to Spotify.

This question has really been bothering me because the loudness difference is genuinely massive, and I’d really like to understand the technical reason behind it.