ALLEXCEPT - How does it work?

Hello guys, I have a question to the function ALLEXCEPT.

As Microsoft says, the function “removes all context filters in the table except filters that have been applied to the specified columns”.

The problem is that I see a different behavior. Ok, let me show you an example to explain it:

The example

I have this list of leaders from US and Germany with their party and the date of taking office.

List of leaders:

Now I want to create a report that shows the number of leaders for both countries.

In this report I will put two slicers: the first one sets the date range of taking office and the second filters the party.

Here is the report:

https://app.powerbi.com/view?r=eyJrIjoiMGVjMTAyYzEtZTNkMS00YzgxLTgwNTgtMmYxYmQyYmYxM2NkIiwidCI6IjUzYThmMDUyLTkyYzctNGMzZC1iYTlhLTRiYjgzZjFjMDQ3YyIsImMiOjh9 

The measures

Here is the measure that counts the number of leaders:

Number of leaders = COUNTROWS('List of leaders')

Now I create another measure with ALLEXCEPT that will show the total number of leaders. This measure should only be filtered by the date of taking office, but not by the party. Here it is:

Total number of Leaders =

CALCULATE(

    [Number of leaders],

    ALLEXCEPT('List of leaders','List of leaders'[Taking office])

)

The problem

If I change the date range in the first slicer, both measure get recalculated. For example, I change the date range to 1998 – 2020, the measure with ALLEXCEPT shows 5 leaders. That is all fine and that’s what it should do.

But if I now select a party in the second slicer, BOTH measures get recalculated again. And that is what I don’t understand. For example, I select Republicans and both measures show 2 leaders. But in my opinion, the ALLEXCEPT-measure should not get recalculated when I select a party!

And another strange thing: If I don’t change the date range in the first slicer (so 1998 – 2021) and then I select a party, the ALLEXCEPT-measure gets NOT recalculated!

So, can you help me to understand this?

With best regards!