I'd always thought that a fully-flexible ticket meant that:-

1. [*]
2. [*]

But to get a refund, you'd have to cancel the ticket, not just the reservation - which are two different things.

For such flexibility, you have to pay a price. And one consequence is overbooking, because the airline can't know until you've boarded the aircraft and it's pushed back that you're actually going to use your ticket on that flight.