Anyone know how UA PNRs are generated?

Two itineraries I ticketed within 15 minutes of each other have the same first two letters and then the third letter of the itinerary ticketed earlier is before the third letter of the itinerary ticketed later. Are PNRs assigned sequentially?

Then again, I ticketed an itinerary about 2 days before those two and have a first letter after the first letter of said two itineraries.