There is a very simple explanation. Ideally if it is 300 yards across that cove and the bullets impacted about 1/2 way across or approximately 450' from the muzzle the CB should have taken .608 seconds and the CCI .274 seconds. The trajectory of the bullets were not taken into account.
The major flaws in this test is two different rifles fired at the same time as close as possible with no reference to the level of the barrels and no timing device.
The horizontal plane (water) is not a good reference for this "test". The impact plane should be vertical for a more accurate result.
But even at this range of approx 150 yards the actual flight difference was about 1/3 of a second if the listed velocities were the actual velocities. That is a significant difference.