This is in no part due to QuickBox. By default Linux installs reserve 5% of the disk... here's a bit more as to why:
Saving space for important root processes (and possible rescue actions) is one reason. If you allow others to log on to your system, via ssh, for example, having these 5% blocks reserved ensures external users cannot fill the disk. Even if you don't allow others to log in to your system, the reserved blocks prevents programs not running as root from filling your disk.
If you set the reserved block count to zero, it won't affect performance much except if you run for long periods of time (with lots of file creates and deletes) while the filesystem is almost full (i.e., say above 95%), at which point you'll be subject to fragmentation problems.
As per the widget, I have never seen behavior like this on any install. Could you perform the following to see if this addresses it.
cp /root/QuickBox/dashboard/widgets/disk_data.php /srv/rutorrent/home/widgets/disk_data.php
Select 'yes' if it as you to overwrite and then do
service apache2 restart