********* Before anything, this subject has been solved thanks to @shroomhead_one and @Shruard7 replies so thanks guys !! And here is the answer :
I almost always compare my masters to others. Preferably big ones.
But this time, I compared my master through WLM Plus (which is Waves' loudness meter) to a song I downloaded (just for the test) though a YouTube to mp3 service. And as @Shruard7 mentioned :
And @shroomhead_one pointed the 'Stats for nerds' which indeed indicate a -5.7dB volume operation from YouTube.
So YouTube actually lowers the volume to match to -14LUFS which is what I was wondering about but using a wrong method until now
********* Here is my original message :
So I've been working like crazy and I'm back to making music. And I can't wait to share my new track.
So as usual, I'm working on the Master before posting it on YouTube. I have an effects rack with iZotope Ozone 10 and WLM Plus by Waves with a preset made for YouTube streaming.
And I found my Master rather low in volume compared to others if I respect the -14 LUFS rule.
So I imported a track by Rok Nardin called "Where is your God now" (amazing track) in my session, and found out it was measured as a -8.7 LUFS long term max...
which is WAAAAAY over the top. 
I watched tutorials on WLM Plus several times and think I'm doing it right but my uploads are way less impactful and loud because I sometimes need to put them down by -4dB to reach the sacred -14LUFS...
I always listen to the whole track with WLM Plus engaged and put the volume down if it ever goes over -13.5 LUFS (which still has the little green check).
So I'd like to know why I see this : Rok Nardin = -8.7 LUFS and it's on YouTube and I still see topics on the need to make our tracks -14LUFS
If anyone knows the answer, I'd be more than happy to know why I'm bothering
Thanks a lot and have a great day
Sebastien