Why are some pencil marks disabled on the number selector?

A. By default, the number selector only allows you to select from the pencil marks, so if the pencil marks are incomplete or incorrect, some choices will be disabled on the selector. In other words, it assumes the pencil marks are both complete and correct, and then only allows you to select one of them.

You can change this behaviour from the options dialog - if the "Allow Invalid Moves" option is selected, you can choose any number from the selector, not just those that correspond to the pencil marks.