By "code" I mean XAML controls and WinRT APIs.
Now, 90% is a high proportion and the assumption therefore is that it should be really easy to use the same code on both and most apps should be able to reuse almost everything.
But what if I'm playing devils advocate?
Surely we can look at that 90% differently.
Let's work on the basis that it means that 9 in every 10 items can be used on both platforms.
This means that 1 in every 10 items doesn't work.
Do you have 10 (or more) controls in your app or use 10 or more of the APIs in the SDK?
If so, then you'll have something that can't be shared across both platforms. At least in part.
Isn't it only the smallest, simplest apps that contain less than 10 controls or use less than 10 APIs.
Two points then:
1. Statistics can be manipulated to skew opinions and support different arguments.
2. Don't assume that creating code that can be used in both a Windows and a Phone app is straight forward and everything will just work the same on both platforms.