How do you know that is was NCL that wouldn't adjust the difference in price, and not a greedy travel agent that didn't want to lose the commission?
NCL is one of the cruise lines that will not lower the price nor give an onboard credit, if the price goes down. There have been several threads in this section discussing that issue.
Silversea is another such cruise line. There are several out there, just as there are a few that will offer compensation.