In the Crossfilter documentation, it states the following.
a grouping intersects the crossfilter's current filters, except for the associated dimension's filter. Thus, group methods consider only records that satisfy every filter except this dimension's filter. So, if the crossfilter of payments is filtered by type and total, then group by total only observes the filter by type.
What is the reasoning behind that and what is the way around it?
The reason is that Crossfilter is designed for filtering on coordinated views. In this scenario, you are usually filtering on a dimension that is visualized and you want to see other dimensions change based on your filter. But the dimension where the filter is defined should stay constant, partially because it would be redundant (the filter mechanism is usually displayed visually already) and partly because you don't want your dimension values to jump around while you are trying to filter on them.
In any case, to get around it you can define two identical dimensions on the same attribute. Use one dimension for filtering and the other for grouping. This way, as far as Crossfilter is concerned, your filtering dimension and grouping dimensions are separate.
