why is everybody astonished ? the build of win seven has started aprox. a half year before vista RTM.
So the question should be: why so late this time
OK, the build-numbering is wrong, because the complete 7xxx builds are reserved for win seven (and sp1, sp2. sp3 .... ), MS might start the builds with 8xxx (its just a guess, don't blame me )
and midori has nothing to do with windows, even some technologies will find their way into win8 as they had already in win 7.