It's done intentionally to make the owners a bit of money. They have direct download links on their website (click show all on download page), avoid the green Sourceforge link.
Yes, all the downloads are hosted on Sourceforge, but Jonnerz is pointing out that the additional links come without the Sourceforge wrapper (the links will have "?nowrap" at the end).
We all wish FileZilla would just drop Sourceforge completely, but at least the non-wrapped versions are still available.