I don't think the non-rev thing is always happening.
I often sit in 6H in the front C cabin of a TATL 777, and keep an eye open for whether or not 6G is empty.
On a recent flight, at T-24 6G and 6D were both unoccupied. ^ After OLCI I noticed that two relatives, same first three letters of last name, different initials, were waitlisted for Y, and for C, and for F.
After I had boarded the plane, they arrived and sat in 6G and 6D.
I talked at length with them (in fact one of them would not stop talking to me!).
Both brothers. Neither were non-rev — in fact both kettles with no status who had never ridden in C before and had no idea how things go in C, how the seat controls work, that you can selected your own IFE, what you can do about food, drink, etc, etc. They had flown to New Zealand in the back of the bus and had no idea why they were up there in Biz from IAD to LHR on their final leg back. I told them they were lucky and should not have been sitting in those seats as there was plenty of seating in Y!
So, if someone can explain all that to me — in particular how they came to be on all three standby lists (and actually Y was far from full so they could have flown in Coach and left me in peace!) when they were no more than kettles — I'll be more than interested to hear about it.