Lex, rex, and offspring

I was pondering the other day why lex and rex (genitive legis and regis, that g can be found in their derivational families) have become something different in Italian. In Portuguese you have rei and lei, in Catalan rei and llei, in Spanish rey and ley, in Romanian rege and lege, in French roi and loi, but in Italian re and legge. This dictionary states that re used to be rege, which doesn't match legge with two g's. Why did one word have one g and the other two?