Ambiguous Drill-Through Behaviour

Hi

I'm relatively new to Power BI and have been spending some time learning the "Drill Through" functionality.

I have a sample AdventureWorks file thatI've been working on that has a Sales page and a drill through page that shows Demographic information.

When I select multiple columns (in this case, months) on my TotalSales bar chart I can't right click and drill through and my drill through button doesn't work (this is expected as I know it needs a single data point):

When I select a single month, everything works as expected:

So here's where my question comes in, why, when I select multiple columns (disabling the drill through from the bar chart and disabling my button) can I still drill through from the card?

...and just to confirm, it brings both months through into the filter on the drill through page:

 If this works from the card shouldn't it also work from the bar chart and button? Why is it disabling my button with an error of "To drill though to Demographic, select a single data point from SalesAmount" when it quite clearly has the functionality to drill through with multiple data points selected?