Systemd bybasses fstab mounting root partition(?)

[Ivan Boro <magwa.man@yandex.ru>]

Looks like subj.
Have a KVM machine with jessie assembled via debootstrap. Fstab states

/dev/vda2       /       ext4    rw      0       0

The partition labeled vda2 is listed in grub.cfg as the root partition 
by its uuid.(via update-grub)

Added a disk to have swap on it. Booted normally, despite the fact, that 
the new swap-disk was automatically labeled as vda and the disk with 
root is as vdb:

# mount | grep ' / '
/dev/vdb2 on / type ext4 (rw,relatime,data=ordered)

So, systemd mounted root partition, whitout looking into fstab.

I don't have much experience in systemd, and now wonder how this "smart" 
mount works.

Ivan Boro