Why does my iPhone's volume button override the set volume in Settings?

In settings, I set the volume for my iPhone in the middle. Apparently the volume button on the side overrules that. Maybe as I am working and have my phone in a pocket on my chest it's possible the side button was pushed accidently so that the next time I turn on my audio book it blasts at top volume in my ear. Does that make sense or is it something else?

[Re-Titled by Moderator]

Original Title: controlling the volume