Well, it can't be, if people are getting it.this is a bad april fools joke.
But for Hookbill - there probably is a pattern, but it would take some mucho extra digging around to find it.
We know that an update is going out to a finite number of phones. Now, because the number of phones able to receive the OTA is known (maybe not exactly, but...) it cannot be a random update. For it to be random would mean that there is an infinite number of phones out there.