On 8.04 and 10.04 package csh correctly adds itself and removes itself from /etc/shells on package installation and removal. That would suggest on your machine either /etc/shells came with /bin/csh listed on Ubuntu's installation, or the csh package was installed and removed a long time ago when it perhaps didn't remove itself from /etc/shells.
On 8.04 and 10.04 package csh correctly adds itself and removes itself from /etc/shells on package installation and removal. That would suggest on your machine either /etc/shells came with /bin/csh listed on Ubuntu's installation, or the csh package was installed and removed a long time ago when it perhaps didn't remove itself from /etc/shells.