Mark, your cancellation policy shows Strict on your listing; that's why your guests are being offered the 50% refund.
As others have said, your guests have two choices to get a mostly full refund (Airbnb will not refund its guest fees). They can cancel online and then connect with Airbnb to request a further refund based on the Extenuating Circumstances policy. Or they can cancel online and then send you a request for an additional refund through the Resolution Center. You will get a request online, which you can accept if you are so inclined.
If your guests call, Airbnb *might* transfer their guest fees toward a future booking, but that doesn't seem to be done systematically.