Exactly. I just don't see why you think the mass of the fuel can be ignored. (not you Kny')

Look at it this way, if you throw a cup full of water towards the air, the water continues upward (despite being under 14.7 psi of pressure from the atmosphere). Then the water comes back down, because the mass is being affected by gravity (acceleration). The water is still at 14.7psi through the entire situation, but the external forces on the way up, and the gravitational forces on the way down, still have an effect.

6psi is just gauge pressure, which is really 20.7psi actual, but it's all acting the same way.