Are you holding wide open throttle during the compression test? Stockish bikes only need around 120-125. Not holding the throttle open could bring it down. You want to keep kicking with WOT till the gauge won’t go up anymore,even 20 kicks is fine. Also the engine should be cold.
If you have low compression i wouldn’t mill the head to compensate,just asking for trouble.
Unless the kicker feels very loose or it won’t start,I wouldn’t worry about testing compression on a fresh build till it is broken in. Your not going to get accurate numbers because the rings have not been seated
after breakin 5 psi difference is still ok,as long as it doesn’t go further.