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Probably has a lot to do with the way the installs now, specifically the fact they're using what amounts to a sysprepped image file with a lot of extra drivers included as a way of speeding up the install process.
Not to mention that the DVD contains builds for both x86 and x64 with all the attendant binaries. Also the single DVD contains all the OS features you could possibly have in Windows (all the way from Home to Ultimate).
If that were the case for this build, why have seperate ISOs for x86 and x64?