Several here have stated many of the factors, including how the gun is held, that play a part in the differences in velocity readings. Here's the main point to remember.
EVERY gun and ammo combination is a combination of many, many different factors and ALL of them have a range of tolerances.
There is a bell curve at work here, and while the majority falls in the middle of the curve, there are results that are at each end of the curve.
Because the majority of results will be in the middle of the curve, reloading data is useful as GUIDELINES, they are NOT hard and fast results that you will get.
If you get exactly what they got, its serendipity, no more, no less. Do exactly the same thing tomorrow and you'll probably get a slightly different result.
That's another part of the key. Slightly different. Do not obsess over a handful or even a double handful of fps variations. Guns and ammo do that.
And for every "rule" you can think of, there is some exception, somewhere. I've seen a shorter barrel gun shoot faster than a longer barrel one. I've seen multiple guns with identical barrel lengths, shooting the same ammo, over the same chronograph in the hands of the same shooter vary in velocity as much as 100fps.
As previously stated, reloading data doesn't tell you what you will or should get. It tells you what they got. The
expectation is that you will get something similar. Anyone who promises you that you will (or should) get precisely what they got is selling something.
Which is the main reason I retired my chronograph decades ago. The actual exact fps numbers are irrelevant to me. I shot enough different guns and different ammo to learn that I will get something close to the published speeds, allowing for different barrel lengths and all the other differences between individual guns and the published data so what matters isn't the speed number of FPS but how my ammo performs in my guns.
Only shooting wil tell you that. Getting as close to humanly possible consistent identical numbers in your ammo specs (all of them) provides
potential, but doesn't tell you much beyond that. Only actual shooting shows you what actually happens with your gun, and your ammo in your hands.
Now, someone might come along and tell you how that prepping and sizing your brass is a certain way, and doing other things a certain way improves group sizes, some might even go so far as to say it "always shoots better" or "shoots better in all my rifles"...
To which my response is, "good for you! How does that help me put a deer in my freezer with my .308 Win Remington 600 carbine?"
The only honest answer is, "it doesn't".
There are just too many factors at work that are specific to the individual gun and the ammo being used to claim otherwise, if you're honest about it.