Member security only needs 1 hierarchy in a dimension to be secured - behind the scenes AS crossjoins between any secured attribute hierarchy to any being used in a query and only returns those where members exists in both hierarchies. This behaviour is called *autoexists*. This is good as you dont have to secure every attrib hierarchy in a dimension.
If your business requirements dictate that the totals should reflect the contributions by the allowed members only, you need to enable a special server behavior called Visual Total. To do so, go back to the Advanced tab on the Dimension Data tab in Role Designer, and select the Enable Visual Total check box