No, it's not a bug. I'll again point out that the wording is ambiguous, but it's entirely as intended.
Free space== Space available for writing to by a 'normal' user. (In other words, how much crap you can stuff on)
The root reserved space can only be used by root, ergo it's unavailable & used from a 'normal' users perspective.
If you enable root as a user for Webmin, you'll see the total free space available including the root reserved. (NOT recommended, opens you to all sorts of holes)
Moving back onto topic, if you have any decent degree of understanding of the command line, I'd recommend setting up a remote SSH shell as opposed to Webmin.
Webmin gives you all the data in a nice web interface, but anything more complicated and it falls over. Torrenting also tends to slow it to a crawl, whereas SSH is less affected.
-Leezer-