Well, technically Linux is "NOT" a UNIX. It uses the Linux kernel and the GNU userland utilities. As such GNU stands for "Gnu's Not Unix" and instead is a UNIX-like operating environment.

Also, while AIX, Solaris, BSD, HP-UX and even Mac OSX are "UNIX" depending on the actual certification from whoever holds the rights to determine what UNIX is, many people lump them together as the tools are similar and the commands are similar in most cases.

In the case of a distribution, that is due to the maintainer's decisions of what to include, how to control the releases, etc.

SUSE, Debian, Red Hat, are all using the Linux kernel, GNU userland, etc, however Debian uses different package management, for the most part, Different run levels and configuration files are employed, and the choice of what goes into a release is left up to the maintainer of each distribution.