quite a mythology about what exactly went on.

There is quite a mythology about what exactly went on. For example, Beeching wrote:

After many calculations and experiments, Sholes established the existing keyboard on which the first six letters are Q W E R T Y, and departed from all previous alphabetical arrangements. He then proceeded to sell this “QWERTY” arrangement of the keyboard. It was probably one of the biggest confidence tricks of all time―namely that idea that this arrangement was scientific and added speed and efficiency. This, of course, was true of his particular machine, but the idea that the so-called “scientific arrangement” of the keys was designed to give the minimum movement of the hands was, in fact, completely false! To write almost any word in the English language, a maximum distance has to be covered by the fingers.31

31 Wilfred A. Beeching, A Century of the Typewriter (London: Heinemann, 1974), p. 40.

In fact the QWERTY keyboard was faster in operation than an alphabetical keyboard (see later) so it is possible that there was an attempt to streamline operation, making some letters combinations slower, others faster.

Beechingのどこをmythologyとしているのかな？

this arrangement was scientific and added speed and efficiency.
This, of course, was true of his particular machine, but the idea that the so-called “scientific arrangement” of the keys was designed to give the minimum movement of the hands was, in fact, completely false!
To write almost any word in the English language, a maximum distance has to be covered by the fingers.

completely false!
a maximum distance
あたり、かな？

there was an attempt to streamline operation
some letters combinations slower, others faster.