Datastore Metrics Mismatch to Online Docs-- doing something wrong?

[TKAustin]

When we query which metrics are available for a datastore (using rbvmomi) we get the following list back:

pm = vim.serviceInstance.content.perfManager

ds = <get a moref to a datastore backed by a NAS>

available_metrics = pm.QueryAvailablePerfMetric(entity: ds)

         #  "241 = disk.used",

         #  "242 = disk.provisioned",

         #  "243 = disk.capacity",

         #  "244 = disk.unshared",

         #  "136 = disk.numberReadAveraged",

         #  "137 = disk.numberWriteAveraged",

         #  "178 = datastore.numberReadAveraged",

         #  "179 = datastore.numberWriteAveraged"

while the documentation:

http://pubs.vmware.com/vsphere-51/index.jsp#com.vmware.wssdk.apiref.doc/datastore_counters.html

http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.wssdk.apiref.doc%2Fdisk_counters.htm...

lists more metrics, e.g.:

* disk.read

* diskwrite

Are we doing something wrong, or is the documentation incorrect? We'd really like to get all metrics outlined in the online documentation  Using version 5.1

Thanks to all who can provide some insights!