there is no barrier in any breed or mix as for NN.

This is pure chance, especially with a small sample number. if the sample number is large, I would suspect there is something else going on, such as non-NN roo also breeding the hens.

I hatch pretty large numbers of chicks every year and it really does match the usual averages bandied about.

Another way to look at it is to flip a coin and count heads vs tails. The chances is about 50% but it can easily seem to skew one way with say, 10 flips. With 100 flips, it will come out close to 50%.