That's why I pointed out the velocities would be from a tight test barrel. If you look at the SAAMI specs, velocity and pressure barrels all have minimum chambers and specified bore dimensions. The length is standardized to 24" for most chamberings, including .30-06. LC test barrels are the same way, AFAIK, so I assume Frankford's were, too. The tight chamber is used because that represents the worst case for pressure from a given charge. Even so, lower actual velocity in a particular Garand will only tell you the pressure achieved is lower than the test barrel had, but that doesn't change the powder charge quantity, which is what the OP is concerned with not making excessive.
FWIW, did you tip your muzzle up before firing each shot? I once shot a fair number of rounds of LC 65 NM ammo through our club Garands. Tipping up to get the powder over the primer increased velocity about 80 fps and caused visibly increased primer flattening as compared to tipping the muzzle down and getting the powder away from the primer. That aside, with normal horizontal loading, one of the club Garands averaged 2495 fps with that ammo, while another averaged 2611 fps. The box said 2640 fps, but actual records show 2708 was measured in a test barrel (see table below). So, a span of 200 fps from test gun to sloppy Garand seems normal. You see the same thing running QuickLOAD. It'll hit some gun's numbers on the nose, but finding one 200 fps different (usually slower) just isn't all that unusual. Chamber size, throat length, lug setback, bore wear; they all take their toll on velocity in actual guns. They are, obviously, among the reasons there is no one best accuracy load for all guns; only ones that are best on average. You still end up having to test for what yours likes best.
This table is interesting from a couple of standpoints. One is the range of IMR 4895 loads used. It's probably a fair indicator of what lot variance was like for bulk purchases. Another is that FA seemed to try to lock in a load and stick with it for awhile. It may all have been the same batch of powder for '57-'60, I suppose.