down your intake. It's a fixed path so they run out of room, bombard the container they're in, and resist having even more air molecules packed in. A pressure gauge reads these forces as a given pressure.
If you take that path and make it extra slippery, they won't be smashed into the walls quite as hard (so the same flow is made from less boost). Or, you can pack more molecules in before they bombard the walls with the same force (so at the same boost you have more flow).
This is why the same boost levels aren't equivalent, and boost pressure is not a determining factor in power across different setups.