Mastering your music isn't something that you can simply achieve by plugging your track into an application and bang, out comes the master. Nor is loudness something that can be seen or measured on a loudness meter.
Those at the top level of there art will know this. But to others that are unexperienced they may be fooled by simply looking at the meters and thinking "yeah that track is pumping, it's ready to go."
Without disclosing all the techniques behind what it takes to master a track, I will explain the above statement.
You might be looking at the RMS meters and they are showing around -9dbFS at the loudest part of the song. You would think "ok great that's around what I should be looking for".
But what is that -9dbFS is actually a measurement of? Energy.
Now, say all that energy is coming for the low end in your master and not from an even distribution of energy throughout the frequency spectrum. You will be reading -9dbFS at the meters but that will not then translate to a "perceived" loudness in what you are actually hearing. Play that track on a smaller system and you will find out what I mean.
What actually gives the listener that perceivable volume is the mid range. This is really where you make a difference to the "audible" loudness of a track compared to simply what the meters are showing.
Yes the track should have a well balanced bottom end, which adds to that audible loudness experience, but without all aspects of the frequency spectrum adding evenly to that -9dbFS you will have a lack in some area of your audible loudness compared to the measurement.
It takes a lot of experience and understanding to bring a track up to an audible mastered level, rather than simply something that is showing up on the meters. You simply cant fool your ears. Meters can guide you but they can also be deceptive.
So don't just go by what the meters are saying, you have to listen!!
An experienced mastering engineer is worth there weight in gold, a poor one is worth nothing and app is worth a few dollars....