Stored Parameters Threshold

I have a question concerning the StoredParametersThreshold setting and theStoredParametersID function. I have a parent report with links to a subreport and passing various parameter values from the parent report to the sub-report. When the parent report is exported to Excel (2007 or 2010), some of the hyperlinks (links to the sub-report) appear to use query string parameters in the URL in order to pass the parameters and parameter values whereas other hyperlinks use the StoredParametersID function.

Does anyone know what the trigger is inside of reporting services that determines when to use query string parameters vs the StoredParametersID function? At first I thought that is was the StoredParametersThreshold setting. After quite a bit of testing, it appears that the StoredParametersThreshold does have some affect on the StoredParametersID function, but does not directly control when the StoredParametersID function is utilized.

Thanks!