I would have a good bet that lots of them will be on LNER Advance tickets from KGX-SVG, because it's often cheapest and always quickest.
The 20:33 to Leeds is a bit of a special case for good reasons.
I searched for a SVG-KGX return next Thursday, out at 08:30, back at 20:15. Interestingly, even without a railcard the cheapest combination is flexible singles, so even the non-split booking engines like Great Northern's will already be offering singles not the Anytime Day Return. This is important, as far more commuters than usual will be looking at two singles instead of a return.
There's only 20p difference in price between GN and LNER return but it is much much quicker by LNER.
https://trainsplit.com/results?From...5&InboundTime=20:15&Adults=1&OpenReturn=false
Importantly,
this is also what Trainline and even Great Northern's own booking engine will offer. GN is not even a fancy splitting site, it already offers singles.
https://ticket.greatnorthernrail.com/journeys-grid/SVG/KGX/2025-12-11T08:00/2025-12-11T20:15/1//NEWx1?departNow=no&realTime=no&searchPreferences=,,,,true&showAdditionalRoutes=no&showCheapest=no&tocSpecific=no
Also notable that if you plan to be at Kings Cross about 20:00, your options for the next hour area as follows, since the GN service thins out.
19:57 1 Change FPK, 31 mins
19:57 Direct, 41 mins
20:00 Direct, 43 mins
20:27 Direct, 41 mins
20:33 Direct, 20 mins (+£0.20) - not cheaper, admittedly, but very close!
21:00 Direct, 21 mins (+£0.60)
I would say most passengers will choose the 20:33 from that list. So this particular service is also a quirk of the current timetable and the quirk of fares that are always cheaper as two singles (at the moment!)
This would all very very easily have been solved by LNER just ceasing to sell advances to and from Stevenage. They'd still get people on Any Permitted on the train but on flexible tickets, they're more likely to choose a different train than wait around for that one.
It would also have been helped by contactless being available at Stevenage from next week. People will not plan the journey on a booking engine any more, they'll just tap and go, and not ever be shown the quick option for almost the same price.
PS if you're a commuter heading home around 17:30, the 17:33 LNER combination is actually cheapest by £4.20. This is a big difference! But that train is anecdotally not crowded, it's not an issue anyway!
https://trainsplit.com/results?From...5&InboundTime=17:30&Adults=1&OpenReturn=false