Huge launch costs for MS if they didn't even manage to come close to breaking even even with record sales. Wow.
With the cost of launching a console it's extremely rare for them to break even, or come close (the PS4's success in this area is an anomaly). Cost of marketing, development/production/distribution etc overall is huge.
Bearing in mind we all consider the 360 to be a success last generation, I'd be very shocked if their losses weren't large during their launch period. In fact I know the Xbox division made a $1.9 billion loss in 2006 - the first year after launch.