So that's an increase of in the 3-car 730/0 fleet of 12 sets (730037 to 730048, 36 cars), and a decrease in the 5-car fleet of 9 sets (45 cars), a net decrease of 9 carriages - but an increaase of 6 driving cars from what was previously ordered.
Which of the two 5-car sub-classes (730/1 - 29 sets, 730/2 - 16 sets) is being reduced - or perhaps the number of sets in each sub-class is being revised, with a net reduction of 9 sets?
It seems like quite a late change to the order (unless this is old news, only recently being made public), with deliveries undeway, and presumably, contracts will have been placed in the supply chain (and are in the process of being fulfilled) aligned to the original order. It would be interesting to understand the financial implications.
Perhaps WMT are taking the long view, looking at projected demand over the expected lifetime of the fleet, and decided that the revision to the fleet size optimises total cost of ownership over the lifetime of the fleet. They must have a great deal of confidence in their crystal ball! Or perhaps, and more likely, the change is being driven by our omniscient masters in DfT?