Sun's operating system versioning scheme has seemed confusing to me, since the same operating system version may be referred to with different version numbers. An explanation of Sun's numbering scheme for Solaris can be found at Sun Versus Linux: The x86 Smack-down where the following explanation can be found:
After Solaris 2.6, Sun decided to change how it named each Solaris version. The next version was Solaris 2.7, but Sun called it simply “Solaris 7”. Solaris 8 is actually 2.8, and Solaris 9 is 2.9. They are sometimes still referred to by the old nomenclature (i.e. 2.7), especially when dealing with porting and software versioning.
A bit confused? I've still got more! Solaris versions are also sometimes referred to as SunOS, and different numbering schemes apply there as well.. SunOS was the original operating system released by Sun in 1981 and is based on BSD, where Solaris is based on SVR4 Unix (System V). The last version of SunOS was 4.1.4, which would make Solaris 2.0 (Solaris started at 2.0) SunOS 5.0. So Solaris 9 is also known as Solaris 2.9 and also known as SunOS 5.9.
The article by Tony Bourke also offers a comparison of Linux and Solaris.
Another good source of information on the naming of Solaris version naming is the Wikipedia SunOS article.