Why are my PMS CMYK Breakdowns different to my PMS book?

Firts of all, there're several Pantone color palettes. For example, Pantone + (Pantone Plus) series are different than previous palettes, and Pantone + v.2 are also different than Pantone + on some values. 

But, more important than this, the values for convert to CMYK changes accordig the color profile you're using. In the same way that you have 2 different books according the paper (for coated an uncoated paper are different vaues) also the color settings of the program will produce different results. For example, under Tools / options / default color management, you will find an option for spot colors: Lab values (for better view on screen), RGB or CMYK values (for a better convert). But also, if you change any other values of the color profiles, the convertion could also change. It's not the same to use USWebCoated than Fogra39/Fogra27 or ISOCoated v2