I'm not sure what was the reason of the refund. It was a mistake of some kind. From what information I could get from the bank, they were trying to charge back the entire sum, but somehow got only part of it.
Maybe exchange rate movements? If you paid in USD the chargeback will only work for the USD amount which could be less at the time of the chargeback. If your card is in GBP and it was in 2016, a difference of 20-30% is easy to explain.
Doesn't look like that because exchange rate fluctuation was about 5%. I think it was some human factor issue - "hey, look at this screamingly insecure transaction for a large sum of money, it can't be right". The chargeback was initiated on my behalf, but they don't have any records of me requesting it. Basically, their response is something like "yes, we screwed up, but at least we screwed up in your favor, why should you worry".
