Datascroller doesn't fetch all rows, it just does one query for rows count. Take a look at livedemo examples for an example of in-memory result set.
Could you send me the link for the example you are talking about. ?
I used this link
http://livedemo.exadel.com/richfaces-demo/richfaces/dataTableScroller.jsf?c=dataTableScroller&tab=info, but it does not talk about the original issue.