So. My favourite explanation is that historically odd-numbered / alternate releases of Windows have been the good ones, and Microsoft want to prepare us for how bad this will be. Foreshadowed here:
Microsoft skips 'too good' Windows 9, jumps to Windows 10 | InfoWorld (April 1, 2013)
Alt: they want to subtly refer to it as Windows X.
I've just thought of this. The real good stuff will be in "Windows Spinal Tap Edition" where they turn it up to 11.