One other factor that has not been mentioned yet in the "velocity / pressure" question is the effect of the burn rate of the powder.
If your powder is too fast, it will create a pressure spike early in the bullets progression down the bore, then begin to taper off. That is why using a powder that is too fast will give you pressure indications, but velocities may be low. So in that case, more pressure does not equal more velocity.
Another way to get a pressure spike is if you have a tight neck chamber which does not allow the case neck to expand and release the bullet. This situation can give you fits with pressure signs, while at the same time producing low velocities.