It is decay. WTI price remaining flat, UWTI goes to zero over a given amount of time. There are consistent examples of where UWTI fails to reach its previous price when WTI is the same price, and the shortfall increases with time. I think someone actually posted a decay percent based on some past quotes.
And don't get me wrong, I love this ETN, along with dwti, ugaz and dgaz. I had 7 trades today spread over all four of those. There is definitely a great return potential, its just that some of the expectations are over the top.