There is a cue, though its a rather subtle one: questions is underlined.
Each top level tab (Main (default), Questions, Tags, Users, Badges, Unanswered) has a different set of sub tabs.
And so on.
It just happens that the Main and Questions tab both have similar views into the system, but they are separate and distinct tabs. The Unanswered tab also has a view into the questions, though that tends to be something that is intentionally clicked as it is further away from the site logo.
One thing to note in the different views is the emphasis given by the tab name:
- Main -> Top Questions
- Questions -> All Questions
- Unanswered -> Unanswered Questions
And thus, the sub tabs for each of these is applicable to that tab title.
Could something be done? Maybe. The simplest approach (IMHO) would be to have a 'Top' tab along with Questions, Tabs, Users, that is equivalent to the site's main page. Thus, when you hit the Main page you would have Top highlighted.