version numbers for those Windows OS's whose versions are not in the name:
Windows 95: 4.00.1111
Windows 98: 4.10.1998
Windows 98 SE: 4.10.2222
Windows ME: 4.90.3000
Windows 2000: 5.0.2195 (the "build number" is different for the different SPs... this one is SP4)
Windows XP: 5.1.xxxx (same as above... also merges business (NT) with consumer(9x) lineage)
Windows Vista: 6.0.6002 (Revision different for the SPs, same as above)
Windows 7: 6.1.xxxx (again... SPs...)
Windows 7 is Version 6.1, but that isn't as catchy as just 7.
There is no technical reason it's called Windows 7. It's purely marketing. Same reason they moved away from calling the OS by their version numbers to begin with- marketing. (Which would sound better in 95, "WINDOWS VERSION 4.00.1111" or "Windows 95"?)
It's purely a marketing thing.