Very informative and interesting results with the short barrel having higher velocity. My guess is with the 10/22 blow back system. The casing leaves the chamber when the bullet reaches at around 14"-16" in a 18" barrel so any energy thats left escapes out into the action and the last few inches of barrel actually slows the bullet down. I bet in a bolt action. The longer the barrel the higher the velocity.
The bullet is long gone before the chamber opens. Anything different would result in a face full of high pressure gas every shot.
Mark