The reason for the "pressure difference" is in the gun/powder.
First the powder, are you shooting the same lot of powder that they tested? No, so the burning rate may be somewhat different.
Next the gun.
The bolt gun in their data has a 14-twist, what is your AR?
Probably a 9-twist, but very well could be a 7-twist; just like the Sierra data.
The faster twists generate more pressure. An over-simplified explanation goes like this:
In the 14-twist the bullet leaves the case and engages the rifling. It slightly slows forward momentum, as the rifling begins the bullet rotating. With a faster twist, the bullet slows more, or the "dwell" time is increased as it must spin faster for the same distance traveled. The acceleration in the direction of spin cannot be instantaneous, so the forward movement is ever so slightly slowed, causeing the propellant to generate more pressure at that point(compared to the slower twist rifle).
