Also to remember, the things presented for NPC and even Boss NPC are just guidelines to help you along the way. It can never be perfect or exact because there are so many different factors that will make an individual encounter easy or hard, like many of the things that @SamWilby mentioned.
There is no catch all b/c everyone is different and there is player ingenuity. This post I made on another thread goes into what to really consider for an encounter as well, which is basically saying that the feats you choose are hugely important too.
In fact, we usually say that the feats you choose are more important than higher attributes. The thing that isn’t mentioned on that post is Action Economy that Sam spoke about and I eluded to with the 3 vs 8 comparison.