ITA uses MIBs (from ESX) to classify servers in the ESX bucket. Specifically it uses VMWARE-RESOURCES-MIB (http://www.oidview.com/mibs/6876/VMWARE-RESOURCES-MIB.html) and VMWARE-VMINFO-MIB (http://www.oidview.com/mibs/6876/VMWARE-VMINFO-MIB.html). If you have Linux box, you can do a snmpwalk and search for these OIDs. You can even try grepping for the VM name in the snmpwalk.
You can also compare the snmpwalk outputs of the servers that are classified correctly, to the ones that are not.
But since the version of ESX is the same, I dont think you will see any difference.