It's doing it as it's set to bootup to the Administrator by default.
If you want to boot into another specific account by default instead, download & install MS' Tweak UI click here.
Then run Tweak UI & navigate to the Logon > Autologon section (in the left pane). In the right pane, put a tick next to 'Log on automatically at system startup, specify the required User name in the box & click on Apply > OK.
Next, reboot to test it out. It should now log into the specified user name's account. Once that's done you can delete any unneeded user accounts. G