Because Worldcon committees come and go, we try to store a copy of the stats document on the Hugo site itself, rather than linking to an individual Worldcon's own copy; however, immediately following a Hugo ceremony, the link, when it's available, is likely to be on that Worldcon's own site for a while. There are multiple reasons for that. One is that we're generally in a considerable hurry to get the results up as soon as we can, and so we go with the document provided at the location provide. The other main reason is that it's not unheard-of for there to be revisions to the document in case errors are detected, and for a while, that individual Worldcon is the place to get the definitive copy. However, most Worldcons eventually shut down their web sites and all of their files end up lost unless they care about archiving their sites and maintaining their domains, like ConJosé did. So when things settle down, we pull what looks to be the final version and put it on the THA site in the hope that there will always be one place where you can go to get each year's statistics.
It's sort of ironic that people assume that we wouldn't keep this sort of thing and are surprised when they discover that we are keeping it when we can get the figures.