Since you're comparing a new book and a 15 year old book, it is possible formulas changed. I know a few years ago Pantone switched to using L*A*B* values. For digital, it definitely makes a difference. A file sent to a digital press using PANTONE 151 C where the color is based on CMYK alternates (ex. a file created before Adobe updated their Pantone libraries) will print differently than the same file with PANTONE 151 C using the new L*A*B* values. In some cases, you can see a pretty big difference on screen.