Na means het, so this way you can see, you will end up with 1/4 visuals , 2/4 hets and 1/4 who don't carry the gene at all (NN or aa it doesnt matter)
You can't tell which are the hets and which fourth doesn't, thats where you get 2/3 and 66%.
N is normal , a is recessive gene. So this is normal x het albino for example....
This gives you 2/4 Hets and 2/4 normals. This is where you see 50% het stuff come into play. Hope that makes it clearer.