Relative mixing levels

Well, I've been mixing an album and started with drums first, got drums all cleaned up and ready to go, then mixed the 1st song, then saved a session template and went for all others, needing only some tweaking to arrange the song. Basically before I used to not care about levels, at least in Metal, they would get squashed anyway. Now I have all my mixes peaking at -3dBFS, it helps a lot when mastering, lots of headroom to play around ^^
 
This is truly bizarre. I don't mix like this at all.

I never solo a track for setting levels. That makes no sense. I only solo a track when I need to set a gate or when I'm first EQing the kick for example to get a nice click. I don't set any instrument at one particular level. I mix with all the parts in and just turn stuff up or down. If something needs to go up more but is in the red, I turn everything else down, or bring the volume up on that particular track with a limiter (more rare). I thought this was normal ¯\(°_o)/¯

And somehow my mixes peak at -6dB naturally with this method.

What do you guys mean you don't care about levels in metal b/c it gets squashed??