Ive had that SAME thing happen 358.
I have an idea but it is a pure guess.The first shot slips down the barrel with little resistance,which would lower your pressure and velocity.The second shot has to run through burnt oil and gunk,raising pressure and velocity at the same time kinda cleaning out the barrel a bit.Then the following bullets have a consistent barrel friction leading to your narrowed numbers.I am speculating, this is not a fact.(my own disclaimer)From now on i run a couple dry patches to get most of the oil out.