This morning I spent £2.50 and Starling rounded up and added 50p to my coin jar goal. However, my account balance updated and had 96p on the end!
I contact CS via live chat and they were equally baffled. They said they would pass it on to the payments team and get back to me.
About an hour later I realised what had happened. Last week I made a purchase in USD, which appeared as £22.95 with a 5p round up. It looks like today, a split second after my purchase was rounded up, the USD transaction from last week cleared at a slightly different rate, which made the purchase £22.99 (4p more).
Glad to get to the bottom of it but it really had me puzzled for a while!