WMI returns different data than System Monitor

Question

I am using WMI to obtain performance data. I want to know how many requests per second are entering into the IIS. I am using the “WebService” object and the “Total methods requests per sec” performance counter.

I have created a script in VB in order to obtain the data from Win32_PerfFormattedData_W3SVC_WebService.TotalMethodRequestsPerSec.

I made several tests and I don’t know why I receive different results depending on having the counter running in the System Monitor or not. It seems that there is some kind of interaction with the System Monitor and I would like someone to explain me why this is happening.

The script does the following process:
•    Refresh the counter.
•    Wait a second.
•    Refresh the counter again.
•    Read the counter data.

The servers where I performed the tests are receiving an average of 200 requests per second all day long.

If I execute the script every 30 seconds without the “System Monitor” running, it returns values in the range of 1200 - 1600 requests per second. If I open the System Monitor and load the same counter, the script retrieves values in the range of 180-200 requests per second. In this last case, the script and the System Monitor show the same values. If I close the “System Monitor” and I execute the script again, it starts to return bigger values like at the beginning.

Does anyone have any idea about this behavior?

This is the script:

Set colitems = _
    objRefresher.AddEnum(objWMIService, _
    "Win32_PerfFormattedData_W3SVC_WebService").objectSet

objRefresher.Refresh

Wscript.Sleep 1000

objRefresher.Refresh

for each objitem in colitems
	if objItem.Name="_Total" then
		WScript.echo objItem.TotalMethodRequestsPersec & ":OK"
	end if
next

Thanks in advance.