You and and your buddies may like Velvia at an EI of 40 but that doesn't mean the film has an ISO rating of 40.
ISO ratings are determined by where a specific density falls on the film curve in a lab under highly controlled conditions, it has nothing to do with where you or I like our exposures to fall on Velvia or any other film.
ISO film ratings are very reliable and quite standard in practice. The wild cards that affect our perception of ratings are typically related to "our" problems.
Meters, like thermometers and other gauges, need maintenance calibration to stay accurate. If I chose to get all my camera meters calibrated as well as my hand held, then they too would all agree.
My developing regime cannot match the quality control of Fuji or Ilford.
My shutters work well and are close, but perfect, no.
My sensibility bout what a shot should look like is different than yours.
EI factors in all these variables, ISO does not.