I read it as a number of adjustments have been made to wafer no....

I read it as a number of adjustments have been made to wafer no. 1.
The balance of the wafers will each receive either one or more of those adjustments, incrementally through the batch in regular intervals to allow for testing of each, in order to determine which changes should form part of the mass-manufacturing method as each will ultimately affect the cost of production (and at a certain cost/benefit will or won't be worthwhile)

Since there are multiple changes being made, the quickest way to assess the array of adjustments is to do them gradually at regular intervals.
The testing period is unlikely to take just 3 weeks.