I took a flight with FR last September, CPH-EDI, with 2 friends. I checked us all in online, expecting to be spread all over the place because we didn't pay. We got 15ABC. Lucky us. In January I flew EDI-BLQ-EDI with 2 other friends, they were even on another booking.
I checked us all in, no expecting to be sat anywhere near each other, but I opened up my browser, got 8A, then logged in to their booking and they got 8B, 8C! On the return, the same again, row 10DEF without paying a penny for seating and opening up the two separate bookings at 2 different times.
Seems to be no logic to it. All of those flights were totally full, although I did check in at the very first moment I could.