Just like on a premium train, like Thalys, etc, you have to reserve some sort of seat/bunk/bed on a night train. A reservaton for just a 2nd class seat on a train in Germany is €4,50. The additional €5,50 for a night train seat is probably because it's a better seat.
For the CNL train from Amsterdam to Munich, with any rail pass (1st or 2nd cl., the extra, Aufpreis, for a seat/couchette-6/couchette-4/double sleeper/single sleeper is €10/€20/€30/€60/€100.
The complete ticket, transportation plus accommodations, is €140/€150/€160/€190/€230.
However, there are a limited number of advance purchase, non-refundable tickets at a reduced rate, €69/€79/€89/€119/€159.
So, in essence, with a rail pass, versus a full fare, the pass is worth €130, but it is only worth €59 vs. the discounted fare.