Merge Wizard

The Merge Wizard has options for selecting which people are to be merged.

For some odd reason, the select Descendants option does not allow the number of descendant generations to be selected. It is all generations or none. (The number of descendant generations can be selected from the Ancestors(!) option but this often catches a wider group than required).

But the select Descendants option does not allow the number of generations to be selected.

Can anyone tell me why?