[1] Historically, róa and snúa belonged to the seventh class of "strong" (irregular) verbs, which was the only class of verbs in Germanic that had retained the reduplication inherited from the Proto-Indo-European perfect aspect.
In Old Norse, the verb sá ("to sow") also belonged to this group, but it has become regular in Modern Icelandic.
The verbs gróa and gnúa (núa in modern Icelandic) were adapted to the forms of róa and snúa by analogy, although they did not begin with s- or r- (their past tenses in Germanic were *gegrō and presumably *gegnō).
In modern Icelandic, the first person singular ending was replaced by -i in all weak verbs, and the ri-verbs followed suit.
The verb sá then eventually became weak, reducing the number of ri-verbs to the current four.