Compatibility view?

I created a project in RoboHelp 9.  It looked good for a while, but when I view it now in Internet Explorer 8, it doesn't look right unless I turn on "compatibility view." 

I don't know what could have changed, and my understanding was that RH9 projects should be fully compatible.

I've since gotten RH 10, and have re-exported the project, but it still behaves the same.

Any ideas on how to ensure that all users see the help files correctly?