Why does the 0709 have a strange calling pattern?
It is a service that has run like it for years in various incarnations and timings, I used to catch it in 2003 or maybe it was 2004, from Goring to get into London, but it looks like the timings are few minutes different these days and it didn't used to stop at Maidenhead when I used to use it either.
It was a train aimed at commuters started by Thames Trains who lived at the intermediate statio between Didcot and Reading. It was known by locals as the train to catch if you were a London commuter and needed to be in the office by 9am, as it got you quickly into London without needing a change. Anyone who lived in Reading would catch an HST so I assume that it why it never used to stop at Reading. Though I recall it was always a few minutes late!
If you didn't catch this train, the options were to go for the earlier train and change at Reading or sit the whole way, or the one after 8am and again change at Reading and never get a seat. It was a bit quicker, but the best bit was it meant you had a high chance of getting a seat.
It makes a bit more sense if you put it in perspective and recall in the days of Thames Trains and before 2000, the majority of Class 165/166 "Thames Turbos" as they were branded, went between Didcot/Oxford and terminated at Reading (platform 7 which is the west side bay platform). If you wanted to get from Goring to London for example you used to almost always have to change at Reading for either an HST or Stopper between Reading and London.
The was a similar return train in the evening leaving at about 17:50, that was fast to Reading then stopped from there onwards to Didcot but looking at the time table the evening has changed quite a bit!
Also, there used to be a HST leaving Paddington around 18:10 that didn't stop at Reading, first stop Didcot or Swindon...one of the two. Plus if I caught a later train and had to change at Reading, there was a non-stop HST around 8:15 in the mornings too that would always cause some confusion as there were always a few who thought it was going to stop!
** edit:
It also used to go on the fast lines too, as you used to pass the slow stoppers between Reading and London Paddington. One presumes that it now stops at Maidenhead either because of the increased number of trains on the lines between London and Hayes and Harlington/West Ealing, or because something ahead of it is now stopping at Slough, or some other train has changed.