www.nsb.no is somewhat indecipherable even for native Norwegian speakers, as NSB is a prime example of continous corporate rebranding and was an early proponent of Newspeak:customer instead of passenger, product instead of train, which sounds pretty common in English, but which sounds ugly in Norwegian.

But the main point still is: whatever reductions and reduced fares you might find, the chance of finding cheaper tickets is invariably easier early!