Hi! While I am not sure I fully understand your use case, I don't think you are missing anything. This kind of analysis / segmentation / calculate metric is not really possible with the product variable. This is because the product string is a 'list Variable'. A segment pulls back the whole product string when at least one product matches the segment. So, there is not really a way to say 'THIS, but NOT THIS' in a product string.
For example, if you had a product string of: "ProdA, B-Product"
-A segment for 'ProdA' would bring back 'B-Product' as well, as both are on the same hit/string.
-A segment for 'ProdA' but NOT 'B-Product' would still bring back this row as 'ProdA' is in string AND 'ProdA' !=
'B-Product', so that criteria is matched as well.
This is a limitation and likely a very good item to be added to the Ideas.