Normally velocity data like shown would tend to indicate to me that your barrel was too clean and took around 20 rounds to stabilize. What strikes me is your desire to wait 10-15 minutes between strings. I don't usually do that and I don't know of anyone that does (not saying everyone doesn't). In testing this can lead to some temporal effects that might be happening. Obviously the temperature of the round has more opportunity to change. The barrel fouling may be sensitive to temperature or humidity. If using an optical chronograph then lighting is changing.
Depending on how the rounds are stored and loaded into the gun can have an effect especially at lower case fills.
While probably not likely it's possible that if the rounds were shot in the order that they were loaded then a scale/measuring issue might be causing this behavior.
My point is that there are multiple factors that might be causing this and it's not likely that we can identify what is causing it from this one test.
Depending on how the rounds are stored and loaded into the gun can have an effect especially at lower case fills.
While probably not likely it's possible that if the rounds were shot in the order that they were loaded then a scale/measuring issue might be causing this behavior.
My point is that there are multiple factors that might be causing this and it's not likely that we can identify what is causing it from this one test.