Markdown is pretty 'dumb' in that it simply uses regular expressions to detect markdown tags/structures and then translates them into corresponding html elements. So when it sees subsequent lines each starting with a '{number}{dot}{space}' it determines that it's supposed to be a numbered list, so it translates that to an html ordered list. The numbers in an ordered list are not there if you look at a page source, they're only being rendered by your browser starting with '1' by default.

With all that being said, HTML5 supports overriding default values in an ordered list, so with additional logic in the markdown parser having numbers out of order could be done.