I'll do something I was warned long ago not to do and assume that the two chronographs were the same distance from the rifle's muzzle, i.e., the distance to the center between the screens was the same ± an inch or so. Even a foot difference shouldn't account for nearly 100 fps difference in velocity.
The velocities given by various manuals frequently varies by a lot more than could be considered mere coincidence, and can likely be attributed to variations in bullets from maker to maker, differences in powder lots, differences in barrels and chambers, phases of the moon, and several other things. If your rifle is giving ~3000 fps on one chronograph and ~2900 fps on another it might be well to check the two chronographs with a third or use something like a .22 rimfire with match ammunition to check its velocity on the two chronographs. The .22 LR match ammo should have published velocity figures and in general for good ammunition these don't vary by a great deal from the published figures.
Also I assume that your quoted velocities are for several rounds at least five and preferably more, and that temperature and other weather conditions were the same for both chronograph sessions.
With the same chronograph set up at as close to the same distance from the rifle's muzzle and temperatures close to the same I've gotten velocities from my rifles firing the same loads that have varied by 50 fps or more when firing at different times. Although I can't say for sure I think that for "sky screens" that the position of the sun may well produce differences in velocity too. The range I chronograph on faces south, and in the spring and fall the angle of the sun can be quite low which I could see might cause variations in observed velocities as well.
If your rifle is zeroed originally at 100 yards the come up to 500 yards may give you a better idea what the velocity is, but even at 500 yards the variation may be too small to discover the answer.
Where's German when he's needed? Probably out doing something worthwhile like shooting.
