Why is the Sample Recipient field sometimes displayed as a Radio Button and sometimes as a Drop Down menu on the Call UI Pages?
How the Sample Recipient field displayed on the UI is depends on the --asf section signal on the Attendee Notes section.
If the --asf section signal is added, then there is a drop-down menu. If the --asf section signal is not added, then a radio button is used. Whether to use the section signal or not is simply a design choice and does not affect functionality.
CRM Help Documentation: Section Signals