All of Avianca's aircraft are N-registered. Like Derg's favorite "member" said, I suspect this was a financing/insurance thing.
Side trivia: Some Aeroflot aircraft have "VP-B" & "VQ-B" (Bahamas) registrations. Rather than pay the massive import taxes for Boeing and Airbus planes, they just never import them. I guess Russia allows foreign-registered aircraft on their equivalent of an operating cert. The Russian-made aircraft have the regular "RA-" Russian registrations.