For anyone really curious about this, 1, 2, 3, 95 and 98 are using the old MS-DOS Kernel. Windows 3 was the first product also released with an alternative new NT Kernel and available as Windows 3 NT.
So then when they continued with this NT Kernel they continued to count the version number like that (at least retrospectively when creating Windows 7).
For anyone really curious about this, 1, 2, 3, 95 and 98 are using the old MS-DOS Kernel. Windows 3 was the first product also released with an alternative new NT Kernel and available as Windows 3 NT.
So then when they continued with this NT Kernel they continued to count the version number like that (at least retrospectively when creating Windows 7).