Here is useful explanation why sometimes you don’t leave Brazil but still your flight is considered international. And also priceless guidelines how to read between the lines when you’re checking your flight at the airport.

Beware of flight listings at the airport which only show the final city in route (which you’re probably not aware of). Always know and check your flight NUMBER, not just the city you’re flying to (it might not be listed). Expect that a more distant city might be the only one listed for your flight, but the plane will still stop at the airport for which you have a ticket. Strangely, international flights are just the opposite, with only the first destination in Brazil shown — even though the same flight may go directly to other cities.

Some domestic flights in Brazil are considered “international,” giving flyers a chance to purchase items at a “duty free” store in the airport. (There may be passengers on board from other South American countries who have not yet cleared customs.) Also, you must go through immigrations and customs again upon arrival, even though you never left Brazil. Foreign travelers on flights within Brazil do NOT fill out a new immigration form, but show the carbon copy of the one completed upon arriving internationally (with their passport and visa stamp).

* From Wikitravel