The possible explanation is in the recent thread (in the allocations forum) about the 159 to FGW for the summer. Since the situation
edwin_m described SWT have started to receive more EMUs - part of the internal cascade is to move 158s and 159s back onto the Salisbury - Exeter route to strengthen certain services.
http://www.railforums.co.uk/showthread.php?t=100009
However it is likely this strengthening isn't happening exactly as planned because the 458/5 introduction is delayed, but by the summer they should have half the 456s in service, and we know that and the pair of 458/5s already in use has released a few 450s, e.g. from the Guildford - Ascot route, and that in turn may have released a 159 or two previously used on Portsmouth Harbour to Basingstoke/Waterloo peak extras. But as I mentioned in that other thread, SWT do have some published plans for the use of the displaced units on the West of England route.