Wt not only tried to stretch the generation, making it longer, they also are asserting it covers a shorter time. because "Jesus" decreed that the generation would include, or see the final destruction (of Jerusalem, the world), but wt writers say: no!, the anointed overlappers will pass away before the final destruction*.
They shorten the time covered, but try to stretch it by packing more (2 consecutively living groups) into it.
* the good thing in this idea they love is, that the goal of all jws, : to survive Armageddon, will not be shared by the anointed GB, F&DS.