The test is normalised so you don't need to get all the questions right.... you just need to get more right than the majority.
Indeed if the test was simple then everyone would get them all right and it would not be possible to differentiate.
Guessing/luck/performance on the day are all factors but hopefully overall those more able at NVR will on average get more right than those who are not.
It's a brutal process. Thankfully we have a few years off before DD2. (and in case you ask... we haven't started preparing yet!