Positive reflection coefficient
Reflection coefficients are limited to the range -1 to +1 for passive systems, so it is certainly possible to get a positive one.  

In this case, when theta is near 90 degrees, you get (1 - X) / (0 + x).  If X is less than 1, this will be positive.  

Over what range did you plot theta?  The horizontal axis of that graph is the log of the sepatation distance.  The left side corresponds to a theta of about 90 degrees and the right side corresponds to a theta of around 0 degrees.