I once helped setting an article in tex (originally written in word) and made the mistake of not noticing that rich text 10^20 got converted to 1020 in plain text. We only noticed post print but nobody else seem to have stumbled on it even though it was at the core of the articles thesis.

They also used the 2 in 10,000 figure when picking out the comparison syndromes. I believe Retts is ~1 in 10,000. It's definitely not 1 in a million.

My guess is the editor somehow messed up the per thousand sign ‱

https://en.wikipedia.org/wiki/Basis_point

edit: My bad. Correct character, incorrect description. It is indeed "ten thousand".

Somewhat ironically, I think you're off by one character here.

Unicode U+2031 ‱ is actually the per ten thousand sign (check its character name).

Unicode U+2030 ‰ is the per thousand (or "per mille") sign.

The one thing worse than off-by-one errors are off-by-zero errors.