The Pantone libraries in OpenColor use the M2 measurement mode, rather than M0 or M1. This is the case in all GMG software products, including: OpenColor, ColorProof and ColorServer.
M2 is chosen for two main reasons:
1) Adobe use M2 for Pantone colours in all their applications. This keeps the GMG software consistent with Adobe products.
2) The use of M2 gives a better visual match when proofing.
You can read more about the different measuring conditions here: