Three things can cause this. The first and most obvious is error in setting up the chrony. Such as setting it too close to the muzzle blast. More docketing gas out the muzzle crossing the curing will register faster.
The second is when you have more barrel time than it takes to burn all the powder. Most common in .22 and smaller calibers. It is actually observable with a 9mm carbine. Barrel friction in this case can slow the round in the last part of the barrel. But this is rare in major caliber rifles.
Last and if I were going to win this argument (by cheating) would be a hotter loaded cartridge used with the shorter barrel. If this is for tv expect the producers to create drama by cheating.
