Having Paging enabled, the Column Aggregates should be calculated based on the data from all of the group's items, even if the group is spread over several pages. Currently, only the data from the visible items on the current page are calculated.
From Q2 2015 You can use IgnorePagingForGroupAggregates="true" to achieve this.
Aggregating only those items on the screen is misleading when the group includes rows on non-visible pages - it makes the user feel like they're seeing the whole group. When I see a group total, I expect it to include the entire group, not just a sampling.