Got a question on tank fills and fluttering gage

Hello, all. I have a simple question regarding the gage on my Airhog 88cu foot tank, being filled from my month old Hatsan Lightning. Every video, forum advice, and personal advice I’ve gotten says to slowly crack the tanks valve open when the compressor has built up the psi to equal or more than the pressure in your tank. Makes perfect sense to me. What I’m not understanding is with my tanks valve shut, the tanks gage needle shouldn’t move, correct? So far, I’ve attemted a tank fill up twice, and I see my Airhog tank gage needle rise slowly with my tanks valve shut tight. One would think I got a leak somewhere in the tanks valve, but after a long storage on a full 4500 tank fill, I have no leaks. 



Tried seeing if someone ever came across this same issue on two forums under the PCP support sections, and never have. Any input, fellas?



By the way, this Hatsan compressor is awesome. 

Thank you