You are looking at pressure the wrong way. Pressure is a byproduct of combustion. The burning propellant exerts energy on the bullet pusing it forward. It is not pressure that drives a bullet to a specified velocity, but energy from the combusting powder. How the powder combusts and how that energy is applied depends on chamber volume.
In a 9mm, chamber volume is very small, pressure goes-up quickly, and powder burns more efficiently. In a 38 Special, the volume is twice as big, pressure builds-up slower, and the powder combusts less efficiently, so you do not get the same velocity per grain of powder in the same barrel length.
To compound the issue, 38 test barrels are vented which simulates velocity loss due to a barrel/cylinder gap. Then you have the difference between barrels, bore condition, twist-rate, etc. which further add to the difference.
4756 is basically too inefficient in the 38 at 13000 PSI.