My 867 shows 533Mhz with the sysctl
I seem to remember this problem on my old Dell laptop (read intel 'speedstep' technology).
That value gets set when the machine is started. If the machine is started on the battery, then the sysctl value would probably be the stepped down speed (533 for the 867Mhz, 667 for the 1Ghz, 867 for the 1.25, etc).
When the value itself gets set the system isn't loaded, so its reading the lower value. Resetting the PMU just makes it forget its energy saving settings, etc. That's all.