Fstab not mounting VirtualBox shared folder

I have a virtual machine here which I use for development, so as not to mess anything up in my existing environment.

I've made a share named "web" available to the guest OS (Ubuntu 12.10 Server) from the host OS (Ubuntu 12.04 Desktop). I have configured it to automatically mount in /etc/fstab with the following line:

web    /var/www    vboxsf    defaults,uid=1000,gid=1000    0    0

However, when I reboot, it's definitely not mounted, the /var/www folder shows the default install's index.html file, not the contents of the shared folder.

If I run sudo mount, I see the mount in the output:

web on /var/www type vboxsf (uid=1000,gid=1000,rw)
web on /media/sf_web type vboxsf (gid=1001,rw)

It seems to also be mounted in a different location which I didn't setup.

If I try to remount all the mounts in /etc/fstab by running sudo mount -a, it still is not mounted.

I can only get it mounted by manually mounting it using mount. Where is this other mount location coming from and why is what I'm doing not working?