Power BI 101- Adding a measure to a table visual causes "duplicated" rows

I guess this is a very basic power bi question but after spending some hours on the internet still haven't wrapped my head around this.

I have the most simple data model ever:

and two calculated measures that count the distinct count of countries in each table:

Country (Geo) = DISTINCTCOUNT(Geo[Country])

Country (Sales) = DISTINCTCOUNT(Sales[Country])

everything is visualized in the following tables:

What I'm struggling to understand is why in table 3, as soon as I add the # Countries(Geo) measure,
the visual starts repeating the rows matching each country from the Sales table to all countries in the Geography table.

I know it works if I use a crossfilter in that measure but I dont reallly understand why?

Could someone please explain what power bi is actually doing here or maybe point me to some documentation/article that deals with this concept?

This is a conceptual problem

It's a frustrating and not particularly well documented feature.

The explanation as to why this occurs revolves around a feature know as 'auto-exist'. I have neither the time nor the knowledge to give a detailed description of that term (see here, for example, if interested), suffice to sat that this feature only applies if columns from the same table are being used as filters: if columns from other tables are filtering the results, as in your case, then auto-exist does not apply, and, as you are witnessing in your example, a cross-join operation is instead employed.

I recall developing a workaround some time ago for such cases. If I manage to locate it I will add to this post.