1s in other ordinals were misread, but two 1s next to each other were misread wildly more often than anything else. My current theory, which I only hint at in the last paragraph, was that "nth" was in the OCR dictionary, "nth" is close to "11th" in pixel space, and no other ordinal is that similar to a dictionary word. Therefore, "11th" gets misread most often.
For 2, 3, 22, 23, read the second part: https://drhagen.com/blog/the-missing-23rd-of-the-month/. (short version: people used to sometimes write 2d, 3d, 22d, 23d.). This also explains why 12 and 13 don't show the deficit.
However, shouldn't every date with a "1" be less common if that is the case? Why 22 and 23?
I think 11 might be somewhat explained by scanner errors if we assume e.g. l2 is corrected to 12 but ll not to 11.
But I guess maybe 2,3,11,22,23 are less common due to people overcomensating for wanting to not pick dates that look not randomly sampled?