Duration in seconds would have it's complications. You have to understand that the sets are dumb. And that's an understatement. They are stimulus based, meaning they don't do anything unless told to, and that includes updating it's display. In the scenario you're describing, every minute a message is sent to the phone with a string for the display. The phone doesn't even know it's updating the time of the call, it's just a string to it.
So, to change that to seconds, there would be two main issues right off. First, the amount of messaging it would add would be an extra burden on the RTC processor, and second, that burden could be interrupted by other processes and wouldn't be guaranteed in terms of timing. So the seconds, may not update every exact second depending on the load on the system and the network traffic.