The short answer is that the results of those sims are very close together. The one recommended by BiB with your custom gearing strategy simulates to 0.3% less DPS than the highest combo that you tested. That is a very close result, and is considered well within the predictive powers of the gearing strategy.
When you create a gearing strategy, it builds a predictive model that will calculate the DPS of any given set of gear without actually simulating it. It won’t match simulations “exactly” - but it will be able to get you so close that there is no practical difference in-game - and it will be able to look at every combination of gear in your bags (of which there are millions or billions) instead of just examining a handful.
What gearing strategies + best in bags does is: test every single combo of gear you have and choose one of the sets of gear that simulates within 1-2% of the theoretical best. A custom gearing strategy will only be valid for the legendary/set items you created it with, just FYI - the error margin goes up if you change those legendary/set items.
In order to simulate even a small fraction of the gear combos in your bag would take prohibitively long. In your case, you decided to simulate a handful of item combos, so you can just lock in the items that simulated highest if you prefer. With differences that small, though, it doesn’t really matter which combo you use in-game.
We’re slowly trying to convince everyone that simulators are really blunter instruments than people have come to believe that they are, but it’s an uphill battle