Via SpaceX: https://x.com/SpaceX/status/1817312715758862485
2 points
I suspect they have so much thrust that the sea level pressure simply is irrelevant.
4 points
The bell is scaled to match thrust though. If you get too much expansion the flow peels away from the inside of the bell and you get turbulence, pressure variations and then usually a big kerboom, especially on startup as you’re trying to ramp up to steady state conditions. I’m guessing the bell is more high-altitude optimised than true vacuum optimised.