No, it's not reduced friction. This is counter-intuitive, but if you reduce friction, you actually lose velocity because reducing friction lets the bullet pick up speed and start expansion of the space behind it earlier in the powder burn. As a result, peak pressure happens in a larger volume of space and goes down. When moly-coating bullets first became a craze, people kept getting annoyed that their favorite load lost 20-50 fps, and they had to add a little powder to get back up to speed.
Several things come to mind, some of which have already been mentioned:
Chrono setup. SAAMI testing standards use an optical chronograph with the midpoint between screens at 15 feet from the muzzle. Two reasons to match that are:
1) To match what's published, and
2) Lots of folks get high readings or low readings that can be remarkably consistent (but wrong) due to debris like unburned powder flying out of the muzzle. A short barrel turns more of that stuff loose than a long barrel, so if you've been closer to your chronograph than that, it could well be part of the problem.
The chronograph is set up on light-colored soil or sand rather than grass on a bright day. This can cause false velocities due to the sensor picking up a light reflection off the ground. Spread a dark tarp out under an optical chronograph if you have this condition.
If the sun is anywhere near being overhead, make sure your sky screen diffusers are shading the sensors from direct sunlight and that their shadow isn't off to the side or in the middle somewhere. In some conditions, particularly if the sun is not directly overhead, some sky screens actually like the sky and work better with the diffusers removed, so you can try this if the sun is at an angle.
If all that is copacetic, then we get to higher pressure. Higher pressure is caused by several things. One is the longer bullet seating deeper into the case, thereby giving the powder less room to burn in. I did this in the GRT software. Alliant's specs say 51.5 grains should produce 2950 fps from a 24" tube using a Speer 150-grain BTSP. I plugged that load into GRT and found that with the case capacity set to 57.91 grains, I got a velocity match at reasonable pressures. Changing the bullet to the SST and keeping the 2.800" COL, I got 2994 fps, and the pressure went up from a peak value of 56,999 psi for the Speer bullet to 61,444 psi with the SST, or almost 8%. With a 16" barrel, the velocities drop from 2950 fps and 2994 fps to 2657 fps and 2709 fps, respectively. So the bullet choice is making a real difference here, but not 150 fps all by itself. I am going to guess the bullet's ogive is also closer to the rifling lands, which can also raise pressure like this and may well account for the rest of the difference. In any case, pressures are higher with this bullet, and I would back the load off 5% and see how that flies.