Why was I charged interest when I paid my statement balance?

So back in December, I missed my payment by an hour. They charged me a late fee and interest. That I understand. Fast forward in January, I paid off my statement balance for that month 3 days early. I didn’t pay the full balance off, only the statement balance. I noticed that a couple of days later they charged me “interest charge on pay over time purchases”. Can anyone explain why I’m charged this even though I paid the statement balance off?