It also depends on where youre at on the phone. If youre looking at the music player then the media volume will go up or down, but if youre just looking at your home screen with no musiic playing then it should be the ringer volume.
You probably have some app open in the background which it is controlling the volume of. Music playing? I have it controlling volume correctly.
Doesn't seem to be anything running music wise anywhere. Where should I look
To check what is running:
Settings > Apps > Running tab