Hello Shotgun. Only cartridge I have ever seen this for myself is with the 22 LR. However, it could happen with other cartridges, I suspose, for the reason you outline. What is the chambering of your '73's? Did you make the velocity measurements from both guns near the same time of day with the same chronograph spacing? Were the velocity standard deviations/extreme spreads relatively low for both guns?
I have '73 rifles and pistols chambered each in 357 Mag, 44-40, 45 ACP and 45 Colt. Using lightly loaded CAS rounds (except for the 45 ACP which are Wild Bunch ammo at a power factor of about 160 in the 1911), the rifles all show at least 125 fps more velocity than the pistols. That is using Green Dot powder which is just a tad slower than Red Dot.
Interesting observation. Thanks for posting.
TR