I agree that that's a rational explanation. However, today we crossed to the fast line after Clapham and followed a slow train into Waterloo, arriving late. I sometimes wonder what goes on in some signallers' heads.
I just looked at Realtimetrains and although for tomorrow and future days it suggests that Reading trains will use P2 at Barnes, for today most are showing through P1 -
maybe they are having a bit of a play with the working timetable. (now answered as a tree issue in simultaneous post by
hassaanhc above.)
Thinking about it more overnight, I guess since the modifications to add the Eurostar flyover the designations fast and slow are ineffective as both up and down directions merge for some part of their route between Queenstown Rd and Waterloo. Also, as said Barnes Junction is a major capacity issue which causes the down direction to be operated contrary to description - historically it really should have been grade separated like many other LSWR junctions...
In the future, if the proposals in the London and SE RUS for five tracking of the main side go ahead (i.e. without CR2) then it could leave the 'Windsors' with only 3 tracks all the way between Clapham Jn and Waterloo, which probably accounts for the planned capacity of the Windsor side always being given as much less than the main side.