This is a very good point. It's why lenses are calibrated in T-stops rather than F-stops over in the cinema world.
Do you know of a web site which charts the T-Stop values for Canon lenses? Do they chart zooms across their focal length too? Most constant aperture zooms are not fixed T-Stop zooms. A lens T-Stop value can also change according to how close it's focussed. I've yet to see a review site which comprehensively tests this criterior. DXO mark seems to have a single T Stop value per lens, which is useful for a prime but not very usefull for a zoom....a 24-70 f2.8 L will need testing at 24mm and 70mm and probably every major focal point in between.