Here is how Morse speed is usually calculated:

If the text is typical plain-language English, the test word "PARIS"
is used. WPM is the number of times PARIS can be sent in 1 minute,
using proper spacing between dits and dahs, letters, and words.

It turns out that the word PARIS and one word space equals exactly 50
"dit times", with a dit time being the length of time the key is
closed for a dit. (A dah is three dit times, the spaces between dits
and dahs inside a character are one dit time, the spaces between
letters are three dit times and the spaces between words are seven dit
times.)

So if the word PARIS is sent 50 times in 1 minute, that minute is
divided into 2500 dit times. Which is 41.66 bps.

The reason for the difference is that there are so many different
timing issues in Morse Code. The elements, characters and spaces are
all different lengths, with the most-common characters (like the
letter E) being the shortest.

73 de Jim, N2EY