Hi, probably because they have different energy systems. the 100m more relies on phosphagen and glycolytic, but the 200m more depends on glycolytic. as much as the glycolytic contribution increases, because of the cumulative effect of lactate, performance will decrease during time.
In Portugal there are big differences not only between the times of 100 and 200, but also in the times of 200. There is a decalage between the best 3/5 and the remaining swimmers.