from this article: https://nhkeasier.com/story/4248/
As I understand it, より has the following usage:
Nより = rather than/compared to N
so that in this sentence it would be something along the lines of:
"Compared to last year, which up until now had the highest, it (this year) decreased by 15 people."
But from the context of the article, I don't think this is right, as this year should have the highest number of people.
Would appreciate if someone can explain where I am going wrong with my understanding of this grammar.