By default, PulseAudio allows an application to change the max volume output to be louder than the one set by the user. I find it annoying that some apps tend to set volume to 100% which ends up increasing the system volume to unreasonable levels. You can prevent it by setting flat-volumes to no in ~/.config/pulse/daemon.conf.
$ echo "flat-volumes = no" >> ~/.config/pulse/daemon.conf
$ pulseaudio --kill
$ pulseaudio --start
With flat-volumes = no PulseAudio uses relative volumes. Each applications sets its own volume relative to the system output.
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