The Antecedent Statement is Affirmative.—ἀλλά is sometimes found after an affirmative statement.
a. The antecedent clause often has a concessive force, and frequently takes μέν ( cross2900). Thus,
τὰ μὲν καθ' ἡμᾶς ἔμοιγε δοκεῖ καλῶς ἔχειν· ἀλλὰ τὰ πλάγια λυ_πεὶ με
b. ἀλλ' ου' (μή) after an affirmative statement often has the force of and not, and not rather, instead of (sometimes with a touch of irony). Thus, ἐκεῖθεν ἀλλ' οὐκ ἐνθένδε ἡρπάσθη
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].