Take a campaign that has three lists associated with it, lists A, B and C. If you have ‘remove duplicates selected’ and the same email address appears in list A and C then you will only receive 1 email. The deduping is not in list order.
There are a variety of queries which take place to remove unsubscribes, apply filters, remove duplicates etc. The data is ordered by surname and then grouped by id and email so it is dependant on the content of other selected data.