That's just how "samma" works, it always goes with an indefinite noun. There are a few others like "nästa" and "fel" that work similarly.
This one trips me up a lot, although it's actually quite logical. "Samma" itself is always definite, so making the noun it's talking about definite is unnecessary. To say "samma kategorin" in Swedish would be rather like saying "the same the category" in English. I suggest just trying to remember "samma" as meaning "the same", instead of just "same". Hopefully with time it will just "sound right"!
If someone were to classify themselves (e.g. a morning person, or a pessimist, or an introvert) you could say, "Jag tillhör samma kategori."
No, when you put att after a verb it's because it's attached to a following verb in the infinitive. But samma isn't a verb.