That FPS variance is within the variance margin of a lot of chronographs. Lighting, axis of barrel alignment, distance of muzzle from sensors...they all affect readings.
A wild guess is that the first set of shots loosened up your piston head o-ring and litterally warmed up any cylinder grease which does cause a piston to move faster (if the rifle/grease is cold).
We had some early season games this year where LOTS of guys seemed to chrony really low...but ripping off a burst of full auto before chronying resulted in a "normal"/expected FPS reading.
But jeez...40 posts into it and I'm starting to be convinced that a weak battery will affect FPS! My FPS must be dropping all day long at a game...my 400fps rifle must not be shooting more than 350 by the end of the day....
What a crock of shit...if you believe that battery power affects FPS in a normal/typical spring gun AEG then you need to give your head a shake and look elsewhere for explanations.