What i am really trying to achieve is to full control the active TabItem by using the combobox as the navigation control.
Here is what ive got so far:
Building off of hughdbrown's answer, I found this post on MSDN that describes your problem as a bug. You can reproduce it with this XAML (which has the opposite problem as your XAML):
As you can see the Length binding works fine except for in the dropdown, where it is going off the TabItem instead of the string inside.
I'm not sure it's ideal for your purposes, but you can get around it by being a little less elegant and reproducing your headers in ComboBoxItems: