There are lots of competing implementations of Markdown since there was never a definitive standard for it.
CommonMark is the closest we have to a standard - supported by a number of significant sites which have fostered the use of Markdown-like syntax (Berkeley, Github, Reddit, Stackexchange, etc).
The CommonMark standard is fairly basic, but strongly defined.
Reference here: 
https://commonmark.org/help/
History here: 
https://commonmark.org/
FYI: __foo__ is actually 
bold according to the standard (as is **foo**), while _bar_ and *bar* are both 
italics.
From what I can tell by the announcement (assumption based on the fact that they mentioned GitHub which supports CommonMark) - XF2.1 will use something fairly well aligned to the CommonMark standard (
@Chris D can you confirm this?)