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 ...