I think it's just relativity. After all, the gas in the Earth's atmosphere is traveling 29.78km/s relative to the sun (~Mach 85). Inside of the nozzle it doesn'

I think it's just relativity. After all, the gas in the Earth's atmosphere is traveling 29.78km/s relative to the sun (~Mach 85). Inside of the nozzle it doesn't really matter that it's traveling supersonic relative to the air outside the nozzle until it leaves the nozzle. I don't think the difference in speed of sound will have much effect in any case.