i've seen a few sources now say that they think the reason the red bull sare so dominant, especially in qualifying is the fact that their double diffuser is compramised by their gearbox / rear suspension.
by moving everything as low as posible not only have they benefitted from a lower centre of gravity but also from a better airflow on the top of the car and through the rear wing. coupled with an underbody thats producing less downforce than their rivals, red bull consequently have a greater proportion of downforce coming from the top surfaces, and of course these surfaces are less critical to pitch changes and crucially are not affected the way underbody aero is by the ride height altering with a low fuel state.
to put it simply, the downforce they generate may not be as efficient or as much as the best of the double diffuser prioritised cars but it is more constant and more predictable.