This here is apparently the original source of the markdown specification, and there it clearly says that this is the correct behaviour: https://daringfireball.net/projects/markdown/syntax#list

Ordered lists use numbers followed by periods:

1. Bird
2. McHale
3. Parish

It’s important to note that the actual numbers you use to mark the list have no effect on the HTML output Markdown produces. The > HTML Markdown produces from the above list is:

If you instead wrote the list in Markdown like this:

1. Bird
2. McHale
3. Parish

or even:

3. Bird
4. McHale
5. Parish

you’d get the exact same HTML output