Expression isn't synonymous with Volume, but it is similar.

Volume allows the primary range (example 0-127)
Then expression can use that range as a second layer.
So if Volume is set to 127, Expression can vary between 0 - 127.

Second Example:
Volume is set to 80 MAX .. Expression can now modify from o - 80 but no more.

So think of Volume (CC7) as the Master volume slider, then Expression (CC11) as a secondary variable that is defined/limited by the Volume settings above it.

This may help in future situations.
Kinda like using the volume on a guitar amp to set the 'volume', then using a volume/expression pedal to control it; you'll never get louder than the amp setting when the volume pedal is full on. You can only reduce from that.

I do not work here, but the benefits are still awesome
Make your sound your own!