To be fair I may have got my time lines slightly out of context? The 1925 did call Surbiton, but the latest ref I have is a Station Working book from 1993, when it was Surbiton, thence Walton and limited stop to Woking etc. Suspect this changed with the 2004 recast, although I do not have any timetable detail for that or for a number of years afterwards. By the time of the station working books for the mid 20 teens, the 1925 was Clapham (pick up only) thence Woking, and although I remember the introduction of the 1923 (8-455 as it originally was) this may have been more like 2007 or so, but can not be sure now.
I think the situation was that when an outer suburban service (Alton/Basingstoke etc) could be pathed to stop at Surbiton in the p.m. peak and demand was seen to need same, it would have been done, if any such slowing of a Down Main Fast service could have been achieved without knocking on to several other Down Main Fast services behind. The problem with the evening peak as I touched on above, the DMF was as good as at saturation point, 2 minute headways being the norm, with a handful of 2 and a half minute HW's in the mix, and any driver who has driven that route and had to swing over to the DS London end of Surbiton will tell you, they will have sighted the first restrictive aspect (double yellow) before passing Berrylands, so will have already 'shut off' by that point and been in coasting mode ready to slow for the DF to DS crossing. May seem a minor point in itself, but that alone will impact on following DF trains, hence Surbiton may have been seen to be poorly served (by non stop services from/to Waterloo) during some busier parts of the day. The same applied to Woking, where again , certain Exeter, Portsmouth trains etc didn't call in the peaks, but did off peak.
PS: sorry, this has wandered OT.