Pretty sure it's because this car is soooo much worse than anything Seb or Webbo or Danny ever drove at RBR. Just as changing track conditions reveal more about differences in driver skill up and down the grid, the harder a team's car is to drive, the bigger the gap in performance will be between teammates. When the car is bad, it highlights both the better driver's ability to adapt and the lesser driver's inability to do the same. When the car is easier to drive, the lesser driver will tend to be closer to the better driver.
Seb is no Max. Not even close. He was never a dominant driver, as has been revealed in his post-RBR career. He had the dominant car on the grid and only had to beat his teammate to win a championship, which is very often the case in F1. He was just able to consistently get a little bit more out of the car than Mark - who was a very good driver, but not a great one.