As far as I'm aware, transitive dependencies are counted in this number. So when you npm install next.js, the download count for everything in its dependency tree gets incremented.
Beyond that, I think there is good reason to believe that the number is inflated due to automated downloads from things like CI pipelines, where hundreds or thousands of downloads might only represent a single instance in the wild.
It's certainly not uncommon to cache deps in CI. But at least at some point CircleCI was so slow at saving+restoring cache that it was actually faster to just download all the deps. Generally speaking for small/medium projects installing all deps is very fast and bandwidth is basically free, so it's natural many projects don't cache any of it.
Beyond that, I think there is good reason to believe that the number is inflated due to automated downloads from things like CI pipelines, where hundreds or thousands of downloads might only represent a single instance in the wild.