Why does Finder see volumes which are not under /Volumes?
Finder will show all mounted volumes, regardless of where it's mounted from.

Open up Terminal (from Applications -> Utilities) and type 'mount' then hit enter. You'll get output similar to this:

alex@smiley:~|⇒  mount
/dev/disk3s2 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
map -hosts on /net (autofs, nosuid, automounted, nobrowse)
map auto_home on /home (autofs, automounted, nobrowse)
/dev/disk8s2 on /Users (hfs, local, nodev, nosuid, journaled, noowners)


This shows what's mounted, and where. I have a disk mounted under /Users and Finder still shows it at the top level.

This should tell you where those volumes are coming from.

Also, try clicking on one and either cmd-I, or File -> Get Info, to see what Finder thinks it is.