I don't think that have something to do with the Joins, It is difficult to give you a help without an example of some data to understand the problem but my guess is that the problem is related with the SUM([I Id Best]) = 206
I think that this field should be a dimension and because of that is a mistake do a sum, if you had partially another that in the sum are equal than the ID then you are adding to the total example one of 100 and another of 106 of other categories.
Instead try to use ATTR (https://kb.tableau.com/articles/howto/when-to-use-the-attribute-attr-function)
Or use the String IF ([Bestellgroup] = "Supplies")
I hope that help you with that.
I tried to reconstruct this behaviour in an example workbook but there everything works fine, that's why I didn't attached anything. If I use a dimension instead of sum([I Id Best]) = 206 I'll get an error that non aggregated and aggregated fields can't be mixed because "new calculation" is an aggregate. May be I could use INCLUDE?