Why, in Visual Studio 2008, opening a VSTO Office 2003 AddIn project gets upgrade wizard?

Recently a customer asks an issue about using VSTO (Visual Studio Tools for Office) in Visual Studio 2008. He created an Office 2003 AddIn project, closed it, reopened it, and he is prompted to upgrade. To make it more confusing, the upgrade wizard says the solution is created by a previous version of Visual Studio….