We base all of our gear recommendations on data generated from the AMR simulator. According to our simulator, the BiB suggestion will slightly increase your DPS.
SimC is a different simulator with its own model of the game. Raidbots runs simulations using SimC. Both models are good, but they aren’t exactly the same and neither model is created by blizzard - so neither model is exact. The DPS number you get in either will be slightly different.
The other main difference for beast mastery is that SimC assumes you will use your pet’s basic attack exactly on cooldown every time. To achieve this in-game, you have to macro the pet’s basic attack to every ability you’d use and spam buttons constantly - this is because of a long-standing behavior with pet auto cast abilities that introduces a slight delay even when the ability is ready. Some players know about this and play accordingly, but many (most?) do not, so we model it as if players are just using the built-in auto cast. It’s also really a pain to constantly spam buttons for this gain