I've always wondered why they do that (though it's not 100% like that, for example the rear portion of the 1000 SOU-Waterloo appears to then form the 1145 Portsmouth and Southsea stopper, and ISTR seeing a carriage working schedule somewhere which showed plenty of other exceptions during the Mon-Fri off-peak) - probably something to do with the timings (as otherwise the fasts would have a turnaround time of something like 35 minutes meaning that you'd have two fasts occupying two platforms).
However it looks like the Portsmouth fasts could interwork easily (with a 15-20 minute Waterloo turnaround) with the Poole stoppers and the Portsmouth via Eastleigh services, meaning you could use predominantly 444s and 442s, with 450s where needed for capacity, on these diagrams.
Then the Portsmouth/Haslemere stoppers could interwork with Basingstoke/Alton etc and share a common set of diagrams with mostly 450s.