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The grand total does not change because the top/bottom filter is a Table Calculation and Table Calculations filter data on the view rather than the underlying data. This is often a very helpful feature (for instance - you want to know percent of total and retain this top/bottom sort of view but need the total to be retained in the percent of total calculation.
You have a few different options that Jonathan mentions but the easiest is probably to just calculate the value in a new view and append together two views into a common dashboard. This is the #1 solution in that list.