If you're full of it, its probably from something else...
Real world velocities sometimes match makers claims. Sometimes they don't.
Too many factors are involved to be able to predict anything, other than that there
will be variations.
I've seen guns with equal barrels lengths, firing the same ammo have over 100fps difference in muzzle velocities. Its not common, but its not ultra rare.
I've even seen a 4" revolver turn in a handful of fps higher than an 8". This is rare, but it can happen. Its all about the individual barrel and ammo "fit". Most of the time, things follow the usual standards, but sometimes, you get a gun that "thinks outside the box", so to speak.
this is why I don't get worked up by 100-200fps differences in claimed velocities. In your gun, maybe you get this, but maybe you don't. You only know for sure if you test (chronograph) them.
This is also why I find it a bit silly for pistol shooters to argue about loads (especially in pocket guns) where the claimed differences are in the 100fps range (or less).
Looking at what an ammo makers says they get with a given load is a pretty good reference, and certainly tells you what they got, in their gun. What you get in your gun is likely to be close, but not guaranteed, and might be a lot different than what you expect. Sometimes, you even get higher velocities than they claim. One NEVER knows, until you chrono them.