Yes, "още" implies something additional like "some more", but in fact, "oще" serves numerous purposes in bulgarian - it could be used as an emphasis on something, including "повече" and its contraction "по-"; could also be used as "yet", "still" and so on. You could take a look in Google Translate and its "See also" for possible word combinations. Just some examples where "още" is not "more":
Още един сложен въпрос./Yet another difficult question.
Трябва ни oще повече .... / We need even more ....
Така е още по-добре. / This is even better.
Още съм тук. / I'm still here.
Oще не. / Not yet.
What else ...? / Какво още ...?
Although, you are right, there are cases where "още" and "повече" are interchangable.
Трябва ми още ..../I need some more ... is almost the same as Трябва ми повече ... /I need more ...