Both headword and initial gloss are neuter verbal adjectives with a suffix -τέος, -α, -ον connoting obligation or inevitability. Both plural (headword) and singular (gloss) are commonly used interchangeably in impersonal constructions, as in the quotation that follows (though the singular adjective is more common in such constructions).
[1] The gloss may derive from scholia to Aristophanes, Acharnians 394 (Web address 1), where the present headword ocurs, but no surviving scholium uses that gloss. cf., however, schol. to Ar. Clouds 131, and iota 740.
[2] Philostratus, Life of Apollonius of Tyana 1.18; again at omicroniota 1.