Now now ladies, it's not rocket science.
The customer was initially offered £20 airtime credit, the customer (as in buying a phone, not using giffgaff as a provider) then told the agent that, as she's not a giffgaff customer, having £20 airtime credit would be pointless. So the agent, being a good CS adviser, refunded her £20 cash instead. Everyone's happy!
See, not rocket science 😉