can someone please explain

I brought my tank to the local paintball store and for some reason they could only fill it to 4200psi. They tried cooling it down under cold water but when they hooked it back up their compressor would max out at 4200psi. Is this a common thing or is it their equipment? Maybe I should try the scuba shop
 
Took my tank to a paintball shop the first time I have it filled. They filled it but like you only to 4100. When I told them what I was using it for the owner said that the air is not filtered totally for moisture. Well I used it with no problems. So the next time I had it filled I got a scuba shop to do it. Not moisture or dirt for people to breath and also filled to 4700. after it cooled it was 4500 on the mark.