That was part of Sun's "people pay more attention to the first number" reversioning. Solaris 2.6 was followed by Solaris 7.
This was partly because of the competitors (e.g. IRIX) were raving the first digit with each major version. When Solaris was version 2.6, IRIX was version 6.
As I recall it, Management had less confidence with things called "1.X" or "2.X" than things called "5.Y". So, Sun went through and started dropping the small leading digit from the versions of their products.
