The basic pattern may appear square, but everyone knits differently. If you knit tighter or looser, use a different needle size or yarn, do more or less rows, etc. it may come out different than the the pattern. Sometimes tossing it in the washer and dryer will help square it up.
It's a dishcloth. It works the same whether it's square or not so does it really matter? I never worry about it since it's just going to clean up spilled gravy and applesauce..
