There is also the issue that caching (e.g. cdn of jQuery or React libraries) can mean that, if you visit a given website every day in a month, you may only down

There is also the issue that caching (e.g. cdn of jQuery or React libraries) can mean that, if you visit a given website every day in a month, you may only download that library once (the first time). Their estimate is definitely lowball for your first visit, but it might be a reasonable estimate for how much is actually downloaded on average.