Seems like Merc’s ability to start to understand the ‘21 PU has unlocked the ability to better setup the car (better setups, a wider range of setups, etc.), curing the rear instability and improving performance in medium and high-speed corners.
Some hardware-related details re: the choice to run the high-downforce RW:
“The engineers of the Anglo-German team reassembled the puzzle piece by piece. One of these is the increase in performance of the German power unit released at Imola, which made it possible to use a high-load rear wing in Barcelona to stabilize the rear and "cure" the tires of that axle from deleterious overheating.”
”The W12 has instead found the right “balance” , as the engineers of the Anglo-German team call it; the 'optimal' set-ups, due to the obvious adaptations to the different circuits, are now easier to search for. Up to Portimao the W12 excelled over the RB16B in the slow. In Barcelona, due to a particularly loaded set-up and useful to protect the rear tires, in medium and high speed corners. All thanks to the performance and efficiency of the PU.”
https://www.formu1a.uno/dallimportanza- ... in-spagna/