saspic hit the nail on the head.
The relationship between velocity and pressure is not simple and direct. A certain powder in a certain caliber in a certain barrel length can actually produce significantly higher velocity for the same or even less pressure than a less optimum powder choice. Take BB's standard pressure 158 grain 38 special load. It is roughly equivalent in performance to the famous Remington FBI load, 158 grain +P. But the pressure is lower. As saspic stated, a less than perfect powder choice will have a high, short spike in pressure. The peak pressure is what winds up being reported as the SAAMI pressure rating. Pressure spikes are less preferred for getting a bullet up to speed than a powder that applies more even pressure at a lower peak. Such a powder (without the high peak) can often drive the bullet to a higher velocity.
Richard Lee discusses the at length in Modern Reloading, 2nd Ed.
As for whether or not BB or DT adhere to SAAMI, I would think they'd be opening the door to hellacious lawsuits if they didn't.
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