At the @scale conference Igor Zaika, Distinguished Engineer, Microsoft Office spoke about bringing Microsoft Office to multiple platforms such as iOS, Android and WinRT.
He revealed that rather than rewrite the suite, which has a code base as much as 30 years old, Microsoft decided to preserve fidelity and functionality by keeping as much of the code possible in a shared C++ code base that would run on all platforms, dealing with basic features such as layout, with small bits of code as abstraction layers to deal with actually outputting to screen.
Due to this effort to keep as much code shared as possible, the Universal apps have 96% of the same code as the Win32 version, and if you exclude the XAML UI presentation code 98.6%.
The Android app similarly shares 95% of the code with the Win32 code.
This should mean the Universal apps, which will come to both Windows 10 and Windows 10 for phones, should offer a high level of functionality and features, and develop in lock step with the desktop versions and versions on other platforms, which is of course important for Windows Phone users who are concerned with missing and limited features.
The whole 50 minute presentation can be seen below.