Keg head pressure dropping?

I've got a few beers in kegs for a party I've got coming up. I'm waiting to carb them until closer till the event, so they've been sitting with 30psi of head pressure on them for a little while now. I rigged up a little pressure tester yesterday, just cause I was getting a little paranoid about my kegs being properly sealed when I go to carb them. So I tested all the kegs and they're all sitting at about 10psi. I've got no detectable leaks and it seems odd that they'd all have dropped down to the same pressure level, since I pressurized them at different times. My thought is that maybe the head pressure went towards starting to carb the beer. Can someone confirm that that actually happens? Or do I have a slow, high-pressure only leak in all of my kegs?

My thought is that maybe the head pressure went towards starting to carb the beer.

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There's no way to put CO2 on a keg and preventing the beer from carbonating.