As stated earlier, it is literally impossible to implement Markdown reliably because the current spec is so ambiguous – and the last official code release was in 2004. Thus, every single implementation of Markdown deviates from “Markdown” in unknowable ways.
We generally define “compatible” as “meets consensus in Babelmark”, e.g. the unavoidable diverging implementations of Markdown (as the spec was ambiguous-by-design) agree most on this set. See for yourself.
This is where the word common comes from, as in “In this set of markup, Markdown is commonly interpreted to mean…”
There is definitely a tiny bit of unavoidable editorial choice in there, since there might be two “reasonable” divergences that you have to decide among. Wherever choices were made we tried to stay in harmony with the original goals of Markdown.