Why does Wokingham get the cast offs?

South West Trains are currently trialling some new class 707 trains as this picture this week at Staines shows. 

The current trains on the Reading route will be phased out to be replaced by? No not the brand new trains. These are for the Windsor route. The Reading route will get the current Windsor trains.

Why can't we on the Reading route not have the new 707's rather than the Windsor line cast offs? The journeys from Waterloo are a lot longer than Windsor too....