I wonder what criteria they use to advertise something as through, if it's diagrammed that way?
Staying on the same line?
Not reversing?
Likely high number of through passengers?
Maximum wait time?
Consistent diagramming each hour, so you don't get a "messy" public timetable with through services some hours and not others?
In the 80s up to 1985/6, for example, the up Portsmouth-Waterloo '73/83' slow was booked 20 minute wait at Guildford, and for a time, from 1986/7 to 1988/9 IIRC, the down '73/83' was booked a massive 24 min wait there. Presumably these were advertised as through, as it was the same line, and the services worked through every off-peak hour so there was consistency.
By contrast, around 1989 the former '93' Waterloo-Bournemouth stopper was split at Southampton Central into two separate '93's, Waterloo-SOU and SOU-Wareham, with a long wait at SOU both ways. However this was NOT advertised as a through service. In some hours, the units didn't work through, but in some they did. So presumably in order to present a consistent pattern of service each hour, they didn't show ANY of them as through, even though in practice some of them (about half?) were.