One of our Escalation Engineers has posted the following blog on the NTDEBUGGING page:
Recently we have found that under some conditions perfmon will use the incorrect algorithm to calculate averages. When reading from log files perfmon has been formatting the values, summing them, and dividing by the number of entries. This issue has been corrected in perfmon for Windows 8/Server 2012 with KB2877211 and for Windows 8.1/Server 2012 R2 as part of KB2883200. We recommend using these fixes when analyzing perfmon logs to determine the average of a performance counter. Note that KB2877211/KB2883200 only change the behavior when analyzing logs, there is no change when the data is collected. This means you can collect performance logs from any version of Windows and analyze them on a system with these fixes installed.