When you build a segment, the system looks at the conditions and then returns all information at that level (hit/visit/visitor).
So you've got a hit level segment with a product code. Adobe is going to find every hit where that product exists, and bring in all of the information in that hit. For example, if a customer placed an order for that product, it will return that product as well as every other product in the same order (same hit).
There is an option to have "exclude" conditions in segments. However, that has some limitations as well. If you say "exclude products that aren't Product 1", it will exclude any hit that has products that aren't product 1, even if the hit does have product 1 as well, so you might end up undercounting the products you're interested in.
The cleanest way to get the data for specific products would be to use the product dimension and filter in the table for the products that you want. Or to use the segment but then also filter the results using the dimension so you get just the products you're interested in.