Volumetric Efficiency is a common term, in use for decades. It is actually incorrect, but has never been corr4ected because everybody has got used to it...
What is being measured is the mass [ not volume ] of air ingested by the engine.
As for volume, the volume of air pulled in is the same for any given rpm; what changes is the density of that air.
Except it doesn't. Engines are throttled so therefore the air ingested is relative to throttle opening amongst other things. That mass changes depending on throttle position and engine demand.
