Weird L1 behaviour...

[Mr T]

Here's the scenario: I was working on this song. Lots of tracks, hot levels...L1 on the Master fader to keep the whole thing at a reasonnable level... I get back to that song some days later and think "Is that L1 really necessary?...". So I try to put the L1 in bypass.
The mix remains the same (as Led Zep would say) in terms of level. So I decide to simply remove the L1. And suddenly I get red clipping (not a lot but that's not the point here...).
What's up with that?... Is that bypass not a "real" bypass?... Has anybody else noticed such a behaviour?... Just curious.

PS: and yes, I've tried to stop playback and hit play again after hiting Bypass to make sure the change was really made...