I'm still absolutely certain that there are two Holds in the northwest, divided by the river. Not just because the coloring is different, but also because Skyrim is supposed to have nine Holds, and I can't see where else the ninth would be.
Hmm... maybe... although I guess it's also possible that 1 hold was lost in the 200 years. Here's the only image I could find supposedly showing the holds.
http://www.skyrim.co.uk/Images/Maps/nexus_skyrim.jpg
As you can see, not only is the shape of th eholds different, but the whole kingdom, so I doubt it's accuracy and was probably fan made. Here, though, they have 2 holds in the south where Falkreath hold is now.
Also, it's a bit weird that as far as we can see, there are no large settlements in the Reach.