Presumably it was all based on some sort of data; however, with Wessex gone, and the university always growing, it seemed unlikely to work.The powers that be probably felt that Darts would suffice for the U2, when they clearly can't. Swapping them for the purple B7s from Weston makes sense as they are the right colour allready for the U2 just sans branding and the 20 doesn't need the capacity of a decker for much of the winter, where a Dart would be more than adequate. 32002 was still plying it's trade in the 20 though yesterday morning.
Maybe, it was a PR move to shut the complaining residents up. "We tried single deckers like you asked and they didn't work so you have no choice now." Conveniently, they were the right colour to go to Weston-super-Mare, which may have been the plan all along!