Office 2007 vs Office 2008

I am sitting at one of the area schools, having just completed training on Office 2007 for the Windows users. We have gone over the three major changes--the Microsoft Button, the Ribbon and the Quick Access toolbar and talked about the new .docx format. The Mac, Office 2008 users have also received the software upgrade (to the 2008 version) and this group has not required any training--they seem to be making the transition much more smoothly.

Why did Microsoft make two such different versions of the software--the Mac version only slightly resembles the Windows version, yet the documents slide back and forth rather well between platforms? This has created quite a dilemma in schools--is anyone else wondering WHY????