My Latin dictionary has an entry:
Varronianus (adj) see Varro. (referring to the cognomen)
As you say, Varronianus would be a regular derivative of Varronius (
http://en.wiktionary.org/wiki/-anus) and Varronius looks to be a derivative of Varro (genitive Varronis).
As for the etymology of Varro, I don't have any information.