Take for instance: ἔπειτ' ἔξεισιν ἐκ τοῦ δικαστηρίου ὁ τοιοῦτος κριτὴς ἑαυτὸν μὲν ἀσθενῆ πεποιηκώς, ἰσχυρὸν δὲ τὸν ῥήτορα. (Aeschines, Against Ctesiphon 233)
C. D. Adams translates as "...having made himself weak and the politician strong". One could just as well have said "having made on the one hand himself weak, and on the other the politician strong", or "having made himself weak, but the politician strong".
δοκοῦμεν δ' ἔμοιγε...παρακινδυνεύειν εἰς τὴν πολιτείαν (Aeschines, Against Ctesiphon 234)
Adams translates as "But it seems to me". He could have said "It then seems to me that..." or "So it seems to me that..." or "And it seems to me that..." and the meaning would be almost identical. So I don't think you should worry too much about this, it is a matter of style too. There are some cases where one should only translate as "but", not "and", or "then", or "so", but in these cases it's obvious from the context. In textbook exercises there is no context so it doesn't really matter, what matters is that you know what its possible translations are so that you can find your own way through an ancient text.