I've opted for disk encryption during install of El Capitan (v10.11.6) on my Macbook Air.
Logged in from my admin account (not root), I added a new user to manage a second iTunes account (which I thoughtlessly created and tremendously complicated my iPhone-Macbook activities).
Amazingly, this user has been added to the first login screen after restart (see image). Logging in from this screen, I can bypass the disk encryption password, view the System and User directories, and run apps installed at the root level.
How can adding a new user bypass full Disk Encryption?