Did you set the pressures when the bike had been parked out in the sun or had just been ridden?
And then checked them again in the garage after the bike had been left in there cold over night?
That would explain the difference but if the above scenario is not true then it is a bit suspect. Plus both tyres would have gone down and not just one...
Back in the summer one of the rear tyres on my car dropped 6psi which I thought was weird so I pumped it up and checked it again a few days later and it was fine. It didn't lose any again until November when it suddenly lost 15psi. At that point I inspected the tyre all the way around and found a screw which must have been letting air out occasionally.
You would see a nail though and any other type of puncture would consistently let air so if you check it in a couple of days you would have lost some. It is hard with bike tyres though as they are so much more sensitive to temperature.
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