Folks need to consider that the len's RATED aperture may in fact NOT be true to its real aperture size. And the light transmission of the lens might be less efficient due to the use of less expensive coatings. So unless the lens is identical in all the testing, it would not be surprising that a meter reading does not match between two different brands of cameras, both with supposedly f/1.8 lenses. Magazines like Modern Photography and Popular Photography used to pose the ACTUAL max aperture (vs. the manufacturer's rating), for example the Olympus 100mm f/2 might really measure as f/2.1, and pass less light to the meter.
I have taken two different f/2.8 lenses for my digital camera, and I have seen about 1/3 EV difference in the level of light seen by the meter, at the so-called same f/2.8 setting when both lenses were set to same FL to capture the same field of view!
Add to the meter comparison issue the fact that the ISO equation also allows variability due to the manufacturer chosen C value and K value for incident and for reflected meter calibration. So even if the same lens is mounted on two different camera bodies, the readings from their meters may not match.
So in practice, while 0.33EV might be the standard of variability allowed within the ISO equation itself, the fact that the lens passes different levels of light due to actual vs. rated aperture and due to transmission variability, adds more range to the variability of results.