This has not been my experience; or perhaps I am not understanding your statement.
I am wondering if you can give a bit more detail on how you arrived at this thought; are you talking snub nosed only... or ...?
Shot to shot variance can be as high as 200 fps with - well - just about any caliber.
The magnums, due to their higher velociteis, are more prone to that large a figure, however, it's still not unheard of with a non-magnum.
A very common and typical hi/low spread is somewhere around 75 fps.
BTW - it's not my thoughts on the subject, it's pure hard fact.
Reload a lot and chrono your loads.
Check any and all sources on the web and/or anywhere else and look at the individual velocities for each and every shot.
I've said this numerous times in the past. I don't get all excited about a couple/three hundered fps for one load or caliber vs another.
Simple, there's a significant chance that two round, fired back to back from the same revolver, can vary by a couple hundred fps.