There are only a few NN grades, 6 of which are right at the forward end of the ship, which is never a good place to be. Although I see there are also 2 midships. From what I can make out it states that these are only available with twin beds ie the beds cannot be set together as a double. So it might be something to do with size/layout and positioning.
Just to further the above: MM are approx 187 sq ft and NN Approx 150 sq ft.
We had an NN cabin the first time on OV1 - P108a - rather long and narrow . This time we had cabin P101 - both are on deck 5 - which was supposed to be an MM upgrade - much bigger room. Both are at the forward end - so you get the noise of the anchor waking you up in the morning ( no need for an alarm!). Previously we have had cabins nearer the centre which are more stable. I have a feeling that NN are mainly on deck 4 - which is the deck mainly used for the gangway - so you will have a lot of traffic outside the door 9 but you don't spend much time in your cabin during the day anyway.
Thanks for the comments, i will know which to book now, pat.