How does Altiris detect user logon with Software Delivery Policies

Hello

I am using Altiris 7.1 SP2 Managed Software Delivery policy to deploy a package. The package is set to run at user logon. On one computer, Windows 7 SP1, logging in doesn't launch the install. Never seen this before and was just wondering what mechanism does Altiris use to detect the user logging on so that it can launch these types of install.

Thanks