Illustrator :: CS5 Profiles Match But Pantones Print Differently

Ok. I have a problem with colors not matching between Illustrator CS5 documents. I am often emailed documents with pantone colors selected in them. Later, sometimes I need to create another document using those same pantone colors. The problem is, these two documents print the pantone colors differently. In fact, on my screen the file colors look different side-by-side. I figured it was a color settings or color profile issue, so I went in and checked both files. Everything was the same (Working CMYK: U.S. Web Coated (SWOP) v2, Settings North America General Purpose 2, CMYK Preserve Numbers (Ignore Linked Profiles)) . When I paste pantone samples from one document into the other, they match what's in each other's documents, but the two documents themselves don't look the same and don't print the same.
 
What color setting could I be overlooking to cause this difference? I don't particularly care if they match on-screen, but it's the fact that they actually print different colors from the same pantone swatch that is the trouble. A blue pantone from one file will print purple from a different file.