There may be a hint in the docs:
"If the current machine contains multiple processor groups, this property returns the number of logical processors that are available for use by the common language runtime (CLR)."
Could that be the issue here?
Found something else interesting:
By default, the pool is restricted to a single processor group (http://msdn.microsoft.com/en-us/library/windows/desktop/dd405503(v=vs.85).aspx), and thus to 64 cores. However, in .NET 4.5 you can set the Thread_UseAllCpuGroups enabled="true" flag.