does this mean that I can use a few tenths more powder and inch up closer to listed muzzle velocities will the pressures follow.
NO!
Pressure and velocity are not linear with each other. They are related, certainly, but its not a straight "one for one" thing. In other words, an increase in pressure does not automatically result in an equal percentage of increase in velocity.
I'm just thinking out loud why would the muzzle velocity be less if the pressures were the same no matter what size Barrel
mostly because longer barrels let the pressure push the bullet longer. While there are situations with measurable variances, essentially long barrel or short, the pressure doesn't push the bullet significantly harder, but it gets to accelerate the bullet LONGER in the longer barrel. SO the bullet goes faster, not because it is being pushed harder but because it is being pushed longer, so it gains more speed from the longer duration of the push.
Some other points to consider is that every gun and ammo combination has the potential to be different, due to the numerous tolerances and factors involved, and even the exact same make, model and barrel length gun used in the testing, shooting the same ammo used in the testing, may NOT deliver the same velocity as the test gun & ammo. It may be less, it may even be more. It may be (and usually is only a small difference) but the possibility of a large difference always exists.
I've seen 3 different 6" barrel pistols shooting the same ammo, vary by 100fps. That's uncommon, but not so rare as to be unknown.
Generally, you won't get "listed velocity" from a 3.5" barrel when the test gun is a 5" or 6" barrel. Increasing your powder charge to make up the difference MIGHT result in a slightly higher velocity, or it might result in just blowing more powder out the barrel with no significant gain. AND it MIGHT also result in raising chamber pressures beyond normal working limits. Don't try.