You are confusing Volume and Level. They are two completely different things. Volume is perception, meaning, you can put two songs on a stereo system and not touch the volume control and one song sounds much louder than another even though the level meters appear to say roughly the same thing.
The art of mixing takes a very long time to get good at and this includes judging your mids and low end to give you loudness. Too much low end will take away headroom which will take away from percieved loudness. Often this low end is all down in an area that can easily be trimmed off with no detrimental affect to the song.
Your meters are showing you nothing but sample value. I don't worry about comparative loudness at all until I have bounced my song out of Logic with a good 3db of headroom. The song then goes to mastering (even if I'm mastering myself) which is when I concentrate on things like overall loudness.
When you are listening to music you are probably listening to completely finished pieces that have gone through mastering etc. I think what you are hearing is completely normal and there's nothing wrong with your system at all. We have all been through this and battle with it on every mix.
It becomes a very tricky beast, this mixing lark!!