Some fields editable in Detail View and others only editable in a full record edit

Some fields editable in Detail View and others only editable in a full record edit

Hey friends!

Curious if anyone knows why there are instances where default Zoho CRM fields cannot be edited on the detail view. For example, in the "Tasks" module, in order to edit the "Contact", "Reminder", and "Related To" fields, you have to edit the entire record. Other fields, however, can simply be edited by clicking the little pencil in the detail view. At first I thought it had something to do with lookup fields, but there are plenty of lookup fields on a lead record that can be changed in a detail view without editing the entire record.

Ideas?