Probably down to the stupid purchasing policy the BRB had when ordering DMU stock for dieselisation projects. Instead of placing large ongoing orders and allocating units when delivered, they instead ordered small batches - each intended for a specific regional project, and notionally optimised for such. So a lot of small orders of non-standard machines. Made worse by the fact that the managers on the ground were happy to mix-and-match sets, and use them away from the area they were notionally purchased for. The result? Units allocated and used on routes they weren't intended for, or optimised for.........The "Oxted" units were said to be to a narrower profile, but not as narrow as the Hastings units, but then ran turn-and-turn about with spare full size former Hampshire units (and Mk 1 hauled stock). So why the different design?
You yourself have commented in the past on the WR habit of using high density DMUs on cross country routes, when they had low density machines purchased for those very routes