Solaris skipped from 2.6 to 7. UnixWare also jumped from 2.x to 7. HP/UX skippped from 3.x to 6.x. In perhaps the strangest version jump of all time, Darwin jumped from 1.4.1 to 5.1, as part of Mac OS X 10.1.1.
It seems Unix vendors really don't like low version numbers. Of course, Windows famously jumped from 3.x to 95, so it's not like they're any better.
Windows 95 was Windows 4.0. There are several parts of Windows 95 that even indicated this in a user-visible way.
Microsoft was surprisingly good at not skipping numbers, until the jump from Windows 8.1 to Windows 10, which also lead to the NT kernel jumping from 6.x to 10.
Microsoft had to skip 9 because programmers are lazy.
A lot of programs refused to work if there was a `9` in the version name.
That was of course because those programs needed something newer than Windows 95 or Windows 98. So they just did a string search for `9` in the version name.
There may be a few programs that wouldn't work on `8.9` or `10.9` if they were released for the same reason.
It seems Unix vendors really don't like low version numbers. Of course, Windows famously jumped from 3.x to 95, so it's not like they're any better.