These are the answers I've heard most of the time.
Couple of months ago I read the Diver magazine, they talked about it....
Why would it suddenly jumped up to become a 3 days check? The reason is that they were doing some extra overnight tests. e.g. After the regulator is checked with no IP creep, they leave the regulator with the cylinder turned on overnight, they suggested people to do it it especially if it has just gone through a service.
The reason is, the valve seats can take time to bed in, after a few hrs under pressure the IP and breathing char may have changed. So after it has been left for overnight, we can check the IP again, check for creeps, cracking resistance and flutter.
The whole thing is repeated for another evening with the cylinder turned off, this is used to test minor leakages, as the pressure indicated by the regulator's HP gauage will drop as the ari in the 1st stage and hoses slowly leaks away. Even the best regulator would have minor leaks, so this test is just to see how fast it leaks rather than checking if there is any leakage at all.