Right so, in answer to my own question just in case anyone else is wondering. After a twenty-minute phonecall I managed to get an answer (all the while very aware of the fact that I was spending far more on the phone call that the 13 cent interest charge I was querying, don't worry).
First off, the guy tried to tell me that the finance charge was the amount of interest levied on my balance from the date the statement was printed until the date a payment was received and that the deferred finance charge was the amount of interest levied during the period from the payment being received until the next statement is printed. Which made absolutely no sense. After trying to pull up multiple other screens (because their new system is not as efficient as the old MBNA one, apparently) he left me on hold while he went off to query something.
When he came back, he told me that the deferred finance charge was the interest on the 23.99 I had charged during the month of April. As I hadn't cleared the full balance at the end of April, interest is then levied on the new charges from the day they were incurred (that's the deferred finance charge) as well as, obviously, on the outstanding balance (that's the finance charge). I haven't seen this before as I don't use that card anymore and it was an accidental paypal transaction that had gone through (because I hadn't used paypal for a long time and forgot that that card was still linked to there). So that's it. He said that the reason for the finance charges to be split up was greater transparency. Still can't wait for this card to be cleared so I can just close the account entirely. Four months and counting!