Understanding expanded tables

Hello,

Please, consider this situation:

Product table, with a one-to-many relationship with a Sales table (following case presented in this sqlbi.com article).

I want to understand the behaviour of using expanded tables. 

If I write this measure:

NumOfProductsSold = CALCULATE ( COUNT('Product'[ProductKey]), Sales )

It will return me the number of products sold, but a distinct list of the products sold, not a simple count of products. 
Why?
Isn't this using the expanded tables of Sales, and therefore the related Product columns of this expanded table? If this was the case the values would not contain only the distinct values.
Can anybody please clearly explain what is going on?

Thanks,