Server side processing with `-api search.return` option not working correctly?

jpu

I am using JQuery Datatables with the SearchBuilder extension with a predefined filter.
Both the search.return and searchBuilder.enterSearch options are set to true:

    $(document).ready(function() {
        $('#myTable').DataTable({
            "searchBuilder":{ "enterSearch": true, "preDefined": {..my predefined filter..}  },
            "search": { "return": true },
            "ajax" : {...my server side access also passing the predefined filter from searchBuilder },

            ...other options set too
            });
        });

So I would expect nothing to be sent to the server until 'enter' is pressed somewhere(?).
Now when my page initially loads I noitice that my server side request is sent twice? I could live with once, but why twice?