When you use ErrorMetric.Undefined you are actually using ErrorMetric.NormalizedCrossCorrelation. You can find an explanation about the different error metrics here: http://www.imagemagick.org/Usage/compare/#statistics.
I don't understand why you get such an odd value, this looks like a bug to me. I cannot reproduce this with my local development version. Are you using the latest version of Magick.NET? And can you tell me the full version you are using? (this includes Q8/16 x86/x64).
Can you add a link to the two images that you are comparing that are showing these odd numbers so I can try it for myself? Feel free to contact me through CodePlex if you don't want to publicly share the images.
I don't understand why you get such an odd value, this looks like a bug to me. I cannot reproduce this with my local development version. Are you using the latest version of Magick.NET? And can you tell me the full version you are using? (this includes Q8/16 x86/x64).
Can you add a link to the two images that you are comparing that are showing these odd numbers so I can try it for myself? Feel free to contact me through CodePlex if you don't want to publicly share the images.