The wiktionary entry for -ων says it's an ending cognate with stuff like Latin -ens, -iens, and gives the feminine as -ουσα. Therefore it makes sense that we get ἀέκων, ἀέκουσα. But then we have ἀπείρων and ἀμύμων, which are the same in their masculine and feminine forms (at least as shown in wiktionary's declension tables).
Is there some way to tell which words are going to decline which way? Or is it instead the case that all of these words decline the same way, but their wiktionary articles are documenting different time periods or dialects?