It's because "mismas" is not describing the cups. In Spanish, descriptions come after the word being described. For example, in English, I could say "green cups", but in Spanish you would have to say "tazas verde". "mismas" means "same", which really isn't describing anything about the cup.
Tea cup or coffee cup, it don't matter. It might be special or it might not be special. Makes no difference. It could be one of those heavy fat lipped ones they have over at the diner. That's pretty special in its own way, in a way. It could even be a styrofoam cup from the fast food joint across town where they serve fries like sticks of wood. Even though it has no handle it is still a cup and not a glass. In any case, a taza is just a cup. Any cup that is a cup. Yup.
Susanna, see my comment above. I do not see why an adverb is suggested here? You are not the first.... Am I missing something. In any case as I said and someone else agreed you DO have some adjective before the noun so we don't have to reach for an adverb hypothesis... I further suggest it is like grande, viejo, etc in that mismo has a different meaning depending where it is, but no-one more knowledgable has commented yet on taht...