At the time these stations were being planned, the idea of anything longer than a 333 was probably thought to be so far in the future that building for longer than that wasn't considered. Especially considering that costs are in issue with any new stations. The replacement of some or all of the fleet with 3/6 car formations has only just been announced so it is understandable that this wasn't done. It does seem rather short-sighted to the casual observer now though.
Utter tosh. Read some of the RUS reports published in the past 6/7 years where the need for platform extensions up to 6 carriages on this route has been made clear.