Or perhaps his data was different than some would expect because he was working with biological characteristics.
Also, Mendel sent his paper out to the top scientists of his time to be critiqued.
“it should be kept in mind that Mendel not only anticipated but also would have welcomed repetitions of his experiments by others. He would not have benefited scientifically, financially, or ecclesiastically from any outright misrepresentation in his work.”
https://academic.oup.com/genetics/article/166/3/1133/6050294
The larger the sample size (number of coin flips), the closer the result will be to fifty-fifty.
Mendel did not have astronomically large sample sizes.
"Based on a large number of statistical analyses as well as the review of several well-known geneticists, there can be little doubt that the data Mendel presented in 1866 corresponded much more closely to the predictions of his model than could be reasonably expected by chance."
Are Mendel’s Data Reliable? The Perspective of a Pea Geneticist