Filesystems – Disk Usage on Specific Filesystem

the file /etc/fstab is executed regardless of the user, I believe, hence I presume that the partitions are always mounted.

Yes, but there is an interaction with the use of ecryptfs. fstab is processed at boot time. The ecryptfs mount is activated at login time, after you enter your decryption password. The ecryptfs mount shown in the question will mask any existing mount on /home/user1, for example. Note, this would be a bit more apparent when you use the recommended findmnt tool to show the hierarchy of mounts. (The mount output is simpler for getting the complete list of mount options though).

So, you need to mount your storage filesystem outside any user's home directory. Be aware that files outside your home directory will not be encrypted, unless you work out how to do so manually. You can create a symbolic link to it if desired (e.g. ln -s /storage /home/user2/storage). The way it's set up right now does not make any sense, and could break other things as well (I'm suspicious about the ordering of unmounts on shutdown).

Note the question was already confusing to analyze since it lacked the full output from the df commands. There's another corner case, that df /dev/sda1 will show the usage of /dev if sda1 is not mounted. If you look at strace df /dev/sda8, you will see that it does not access the device node; it looks up the directory from the mounts, and then calls statfs("/home", ...). I would prefer to run df -h /home myself...

...I don't necessarily expect the output would be any less confusing when there's an over-mounted filesystem like this though. The omission or - result for sda13 (depending on -a option) suggests that df has some code to detect the overmount case, but it's not working in all cases. EDIT: reported upstream.