There are numerous proprietary score algorithms out there. The newer ones seem to have fixed this bug by factoring history of closed accounts but many online “free credit report” services still use the old ones.
Old algorithms would often penalize account closure due to sudden reductions in average credit age, available credit, or credit mix (any of which might apply to the OP, but especially if that car loan represented a significant portion of their credit history).
Likewise, they would sometimes reward new debt if it significantly increased available credit or added a unique credit type to the mix. For example, a first mortgage could bump a credit score by 30 points or more, even though the individual is no more credit worthy than they were before.
Regardless, I think a good thing to keep in mind is that banks tend to maintain their own internal scoring systems. So not only is there no such thing as “THE score,” but the scores people are referring to when they say that are mostly just one credit bureau’s estimate, based on their proprietary rubric, of how a lender MIGHT see a potential borrower’s likelihood of default.