πλεῖστον as adverb?

Greek Learning Greek
1

…καὶ γὰρ μοναρχικὰς καὶ τυραννικὰς ἤδη τινὰς τεθεάμεθα πολιτείας, αἳ πλεῖστον διαφέρουσαι βασιλείας παραπλήσιον ἔχειν τι ταύτῃ δοκοῦσιν…

…for we have had before now examples of absolute and tyrannical forms of government, which, while differing as widely as possible from kingship, yet appear to have some points of resemblance to it…

OK, this kind of thing is really starting to annoy me. διαφέρουσαι is intransitive. πλεῖστον modifies it. But πλεῖστος is a superlative adjective. In order to get the corresponding superlative adverb, it should have an alpha ending. So what gives?

Thanks in advance.

2

The plural doesn’t appear to be a form used adverbially, at least according to LSJ entry.

3

Thanks Nate. As much time as I spend with LSJ, you would think that I would check it before bothering you folks! Actually they do mention the plural at the bottom. But you are right that most are singular.