Airbnb never charges the security deposit upfront. The security deposit is only charged if you make a claim after the guest has stayed.
If you charged a "security deposit" through Airbnb, I assume you made a change to the reservation and added what you thought was the secuirty deposit to the their reservation. Because it was added to their reservation, it was subject to Airbnb's fees and any relevant taxes, which is where the $80 may have gone.
Of course, this is all just assumptions since a security deposit is never charged up front through Airbnb.