My point was you've got two sources for some music, even if they're coming through the same device, you can't compare what you're hearing and make an assumption about the "loudness" of the original source material based on listening, or even a measurement because you don't really know what the radio "station" is doing to the material, and you don't have any way to be certain that the device itself is going to treat your [MP3] audio the same way.
If you want to compare your mixed/mastered audio file's loudness against something, it has to be against another audio file. Either download music and compare it to that, or rip a CD track. (They will likely be different even for the same song, which is why mastering engineers often/usually master a song differently for different targets.)