Id go with them being cautious and not wanting a repeat of the PS3, launch titles arent going to need piles of ram, so they want to set aside enough so that future additional functions have space to breath, lack of resources and bad planning is why ps3 never got universal music / voice chat.
But its the same for xbox too, only they are more likely to need ram for OS, i would not be surprised if a year or so down the line when OSs on both systems has matured, SDKs are shipping with documentation stating 6gb available, 512mb managed, 1.5gb os, and 2gb reserved for xbox one, their current footprints are just microsoft and sony giving themselves breathing room for their platforms to mature, DO NOT assume it will stay like that forever, because it wont.
Whats funny is, my GTX780 has 3gb of ram, from reports the xbox and ps3 will both roughly have 5gb of ram.. its more than enough already for 1080p video output, if they were aiming at 4k then fair enough but theyre not
