This article is from the RSC/UKSC Cricket FAQ, by Ganesh with numerous contributions by others.
You're assuming it's decided by some objective method,
rather than some cricket journalist or statistician
simply going by their own fallible opinion of speed.
Bowler W____ (classified as fast medium)
bowls faster than Bowler G_____(classified as fast)!
How can one only be a fast medium yet the other a fast bowler?
See John's explaination above.