How is interpreting "shirts without stripes" as "I wish to see shirts that don't have any stripes" guessing?

I would venture to say that most people (meaning your use case is in the minority) who type "shirts without stripes" want to see results showing shirts without stripes not results "containing words "shirt", "stripes" and "without".

I think what is happening here is that you know how search engines work and so you are conditioned to expect them to do what they're doing.