My suspicion is that (at least for most of those people) they just ran out of time before they got to that one and so it was automatically marked wrong.
The words appear in randomized order every time you take the quiz, so it's quite plausible to me that it would appear in the last 2 or 3 (or whatever) words for enough people who also went sufficiently slowly through the first 16-17, to make up a good portion of that 9%.*
Plus a few that just weren't paying close attention.
I never saw the word playwright, therefore I thought it was misspelt. This way I got 18/20
The words appear in randomized order every time you take the quiz, so it's quite plausible to me that it would appear in the last 2 or 3 (or whatever) words for enough people who also went sufficiently slowly through the first 16-17, to make up a good portion of that 9%.*
Plus a few that just weren't paying close attention.
* (Actually 8% now, as I take the quiz.)