Over the past couple of years, FCC has increased on-train RPI substantially, on all parts of the network, and with no apparent pattern. You are as likely to find an RPI on pbo-st neots as you are Hertford loop or welwyn terminators
I always thought RPIs were given schedules and trains to do, but it seems that (given they could get held up dealing with someone at any time, missing the next train) they are given a pretty free run - and must simply log down the trains they were on.
The ones hungry for PFs (the ones giving high fives to each other and comparing notes) will presumably go for the trains where they know they get easy results, while the lazy ones may go for services where they can walk through and find nobody without a ticket.
For the record, for a number of years I combined a paper ticket with an Oyster (Paper Hatfield > Z456 and Oyster Z1-3). Thus, every day I would touch in/out only at one end - having the tickets combine mid-journey.
That is ultimately what got me 'caught' by my favourite RPI who reported me for fraud, held me for over three hours and made me buy a ticket to cover the bit between Z3 and Z4. And we all know how that turned out (apology, case dropped, bulletin to all staff and a small (£50) voucher as compensation - but no refund of the ticket I was told to buy that I obviously didn't need.
To this day, he's the only RPI that ever checked my ticket thoroughly and every single time he came up with something, he was wrong! But that's not for discussing in this tread. However, suffice to say, not everyone will be up to no good - but it's quite hard to imagine such a fraud being detected, at least with anyone holding an annual Gold Card because staff usually give them nothing more than a cursory glance to check the date (and from the distance, it would be near impossible even with good eyesight to read more).
They don't check the validity (origin/destination) nor the photocard number. If they did on the trains I usually took, they'd ask why my ticket ended before the next station.