The Discovery 1 had CDL so the transfer boxes made for them all had it. When they built the Discovery 2 they decided that the new traction control system meant that it was no longer required, so they started building transfer boxes without it.
However, they still had a whole load of the Disco 1 boxes, so they fitted them to the early Disco 2s.
The earlier the car the more likely it is to have a CDL box. No one seems to know when the cut off was exactly, as in all likelihood, when the new boxes arrived they were probably fitting both on the same assembly line at the same time.