There is more than just barrel length to consider. There is the barrel itself, and the amount of difference in velocity.
Different barrels, even ones identical in length, can produce different velocities.
Usually the longer barrel delivers a higher velocity, but this is not
always the case.
The fit of the barrel (bore) and the bullet make a difference. A "tight" barrel may deliver lower velocity than a "loose" one. I have seen a 4" revolver actually deliver slightly higher velocity numbers than an 8" one. Not usual, but not impossible, it depends on the specific gun and ammo used.
Also, there is the amount of the difference. 50fps or so is within what I consider the normal range of diffference between different guns. I have seen 3 different guns, firing the same ammo, and with the same barrel lengths deliver as much as a 100fps spread in velocity. This is uncommon, but it does happen, depending on the specific guns (not the models, the individual guns) tested. Usually the velocity spread is 50fps, or less, but not always.
The idea of the round "running out of steam" in the longer barrel, because it is desgned for a pistol seems logical, but I don't think that is what actually happens. Yes, one can reach a point where all the powder is consumed, and there is no increase in the pressure pushing the bullet. And, in theory, after this point, barrel friction will affect the velocity more.
But nothing I have seen indicates that the bullet will slow down, rather what happens is the rate of velocity increase per inch of barrel goes down. And, this exact number will be different for each individual barrel.
Chronographs are neat things, and fun to play with, now that they are affordable to most of us. And (good ones) seem pretty accurate. One thing they do show us, if you use one enough, with enough guns, is that each gun may be a law unto itself.
Generally, like guns shoot in a similar group, but there are individuals that may be faster, or slower, with the same ammo. Sometimes the difference is significant. Most of the time, it isn't.
Published data is only a rule for the gun(s) and loads used. Those individual guns. For all other guns, its a guideline, (Arrr!
) and while usually close, may not be what your actual gun and load are delivering in your hands.
A shorter barrel being faster than a longer one isn't typical, but it happens, sometimes.