It's not as weird as you think. Linux (more accuratly, GNU on the Linux kernel) is one of the few UNIX-like operating systems completely written from scratch.

Solaris, BSD, and many other UNIX-variants, were based on the original (the only) UNIX by AT&T. It was a process of evolution. Parts of the UNIX kernel were taken, altered, extended, mutilated, and what not to eventually become something different.

Linus Torvalds started Linux with nothing more than a bare editor screen. Every bit of the Linux kernel was written specifically for the Linux kernel, although almost everything was designed to be compatible with the original UNIX and emulate much of its features.

So it's more like saying "A dog or an Aibo".