change start page for project?

I'm using RH8 and trying to update the start page for an existing project. I opened the properties and here's a pic:

you'll see the field for Default topic is greyed out. but what's interesting is that the topic named in the field is not the opening topic for the project. So, what I'm asking is: why is the default topic a different topic than what is the opening topic of the project? second, how can I change it since it's greyed out?