Confused about peaking and dB levels [FL]

I'm on the wrong computer right now so I can't provide screenshots more than this but here:

If you look at the level on the master buss I know that i'm trying to get as close to 0dB as possible and not go beyond to avoid clipping and that.

I use voxengospan as a spectro analyzer.

Whenever my tracks are running at 0dB on the FL mixer master buss they're usually at like around -12 to -21 on voxengo.

I don't understand this, should I be aiming for 0dB on the FL mixer or voxengo? My ears appear to tell me that the FL mixer is correct... If however I should be looking at voxengo for reference then how do I raise the volume more? I've maxed everything on FL and it won't get to 0 on vexengo... just keep layering?

Some clarity would be much appreciated.

And just for the record. How does this volume thing exactly stack when regarding different buss channels? Say i've got 4 channels on my mixer going about halfway it'll usually start going beyond 0dB on the master. How does this work. I don't see why different noises on the sound spectrum would cause an increase in volume..

How this is sufficient explanation of something i've been pondering the last couple of months and never bothered to figure out.

Thanks in advance.