By default, PulseAudio allows an application to change the maximum output volume to be louder than the one set by the user. I find it annoying that some apps tend to set the volume to 100%, which ends up increasing the system volume to unreasonable levels. You can prevent this 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 application sets its own volume relative to the system output.
References: