Differences in Total Capacity (MHz) reported by vCenter

I'm still currently browsing through the documentation, but hopefully somebody can provide a bit of insight.  When I am connected to our vCenter server and looking at the Performance tab of an individual host, I have the ability to view the Total CPU Capacity for that host in MHz. 

Each host has two processors with 6 cores/processor.  So 12 physical cores at 2 GHz each.  So in theory, the total CPU capacity for each host should be right at about 24,000 MHz.  When I retrieve this metric from each host using a PowerShell script, it returns the expected number (roughly 23,988 MHz for each host.  However, when I look at the performance charts in vCenter for each host, it states that each host has a total CPU capacity of 21,520 MHz each. 

Anybody have any ideas why these numbers are different?  Is there some sort of overhead that vCenter is subtracting from these numbers when viewing the performance charts?

Thanks!