I emailed you back directly, but posting my reply here for others
That is a new one. I tested it myself and agree that MATERIALS does not reflect properly when you select the group, but it does when you EDIT the group and select the textured surface. Also, if you explode it, and regroup it, it displays correct area. When I tested it by LAYER, and used the Layer’s SF for calcs, it worked fine after scaling. I tested it as COMPONENT, using area as attribute and it worked properly.
Seems like only MATERIALS is affected somehow and that is a question for my programmer and I shall ask him. In the meantime, would it be possible to use one of these other methods? (do not know what your dynamic component is)