Just being speculative here. Given that it is going to be a simultaneous release and the Windows release is going to be mainly streaming it using App-V, maybe they are porting App-V to Mac and then they will have a Windows & Mac release which are 100% compatible with each other rather than having to rewrite everything again for the Mac.
Also the fact that they are going to backport Skydrive into Office 2011 rather than make it an Office 2013 feature suggests that they are going to keep Office 2011 as the supported "non-streaming" version, and Office 2013 as the streaming version.
For those who use Macs, you got to admit that the quality control of Office for Mac has been slipping over the years (lots of bugs), maybe this is the future, and Mac users will finally get a full blown Outlook with feature parity to Windows.







