Thanks for the replies. I thought the slow Desiro doors might have had something to do with it!
It is also down to history. The Hounslow Loop used to be a mix of 423s and 455s, which were replaced by 450s on most weekday services. As 450s had some sort of SDO facility, it allowed sets of more than 4 cars to be able to call at the short platforms at Virginia Water and Chertsey.
It is a common misconception that *all* Hounslow Loop services are 450s or 458s. On weekdays there are still 4 services (used to be 6 until December 2015) that use 455s (8 cars as they're planned to not go near the two problem stations), and until December 2015 almost all weekend services were still single 455 units.
Even the Windsor & Eton Riverside services, which are normally all 450 or 458, instead get 455s if diverted via Kingston or sometimes if Hounslow Loop needs longer services on weekends due to events.
Until the 10 car extensions you couldn't fit a pair of 450 units at New Madden, Norbiton and Hampton Wick (plus several other stations on other suburban lines), meaning only the front 4 coaches opened, as platform lengths were that tight.