If you have a document written like this:

1) list item 1
1) list item 2
1) list item 3

the Preview pane shows it as

1. list item 1
2. list item 2
3. list item 3

but the PDF export shows all the numbered list items as "1".

If formatted like this:

1. list item 1
1. list item 2
1. list item 3

both the Preview pane and PDF export format the numbers the same (as "1", "2", "3", as expected).