You have unfortunately fallen into a bit of a trap and you are confusing the WebHelp start page with the Default Topic.
When you want to open WebHelp, you point at the WebHelp Start page. THAT is the page that is typically named Index.htm and it's listed in the General section of your Single Source Layout properties as shown below:
When that page loads it triggers a cascade of events. The last event that happens is that the Default Topic is loaded into the Topic pane and presented to the user. The Default Topic is configured in a different area of this same dialog. I think. I no longer have RoboHelp 7 to look at. My screens are from RoboHelp 10. So you may see this on the first of several screens of the same dialog. Normally you have to click a button and browse to the file you wish to nominate as the Default Topic.
Essentially, you have named your default topic to what you really intended for the WebHelp Start Page to be named.