I am not working for MF anymore but let me answer this one as I know about it more.
MF's data format does not support all OSM road types - they are somehow mapped to TomTom data type.
HW renderer has really fast road rendering (I think after revision in Android Navigator 2.2) which uses 8bit stencil buffer and cannot render more than 7 road colors (if there are more colors on screen that it would cause some glitches).
Because of the limitation of HW renderer was scheme editor adjust to edit more road types as one road. However you can modify which road types are merged in scheme_editor.xml in the Navigator folder and after the change you would see in scheme editor more roads. However this xml is overwritten with every Navigator update.