Old Norse Adjective

I have found in an Old Norse grammar that adjectives were declined
weak after the strong declined numeral "einn". this word was later to
become the indefinite article in modern Scandinavian languages.
Adjectives that follows it are always declined weak inn modern
Scandinavian, but I find it strange that this was true also in Old
Norse since "einn" was declined strong. This suggests that the
distinction indefinite/definite had become more important than
strong/weak allready in Old Norse. Anyone know if the grammar is
right?

Haavard