Because it is also a very good setup that is extremely close in value to the wrist/ring setup.
Our tool is set up to rank every combination of gear, not just the most popular with the most popular talents and the most popular relics. That is a lot of combinations of gear, so there will inevitably be a little margin of error. It is a trade off of speed for accuracy, just like the simulator itself – the longer you let it run, the more precise it will get.
We consider the margin of error on the optimizer to be well below what most (any?) user will be able to “feel” in-game. Use the wrist/ring if you like it better. Or use the helm/ring if you like that better. You’ll never raid enough to figure out which one averages higher for you personally, but you will be able to figure out which one you like better.