You're getting what you see on Sheet 2 because Tableau is cycling through the regions and doing what your calc calls for. Simple enough.
But you are actually getting 1-through-5 marks on the sheet, depending on how many regions you selected.
You want to get it down to one mark, and then you can control "ALL" or a list.
In sheet 2 I made my own calc. See [List]. This uses the PREVIOUS_VALUE function, and as it cycles through the regions, you can see that you get a growing list until it gets to the last region.
Also look at two other calcs. [Regions in data] and [Regions on Sheet]. These are LODs that tell me how many regions are in the original data (uses a FIXED LOD), and how many were actually selected (Uses an EXCLUDE LOD.)
Go to Sheet 2(2).
Here I made [Full List]. It grabs the biggest of the 5 marks, and shoves it on all the marks. Play with the filter to see how it looks.
Go to Sheet 2(3).
Here I added another calc called [index]. This gives me a 1-through-N count of the marls. (I took [List] off the sheet at this point. I don't need it displayed.) On the next sheet I'm going to move [index] to filters and just display index=1. We only need one copy of the list. This is how I get one mark (mentioned earlier.)
Go to Sheet 2(4).
I put index on filters. I also made a new version of [Full List]. In there I set its value to ALL when the two LODs are equal. All Regions are selected, so set the list value to ALL. Otherwise just grab the [Full List] value. Play with the filter and see how it looks.
As you noticed in your original Calculation1, you need to run the table calc along [Region]. I had to do that to all the table calcs here too, including [index].