CMTime in AVPlayer's addPeriodicTimeObserverForInterval: callback never reaches item's duration

I use AVPlayer's -(id)addPeriodicTimeObserverForInterval: queue: usingBlock: method to update UI up to playback progress. However, my progress bar never reaches end.

CMTime duration = self.player.currentItem.asset.duration;
float totalSeconds = (Float64)(duration.value * 1000) / (Float64)(duration.timescale);
NSLog(@"duration: %.2f", totalSeconds);

__weak __typeof(self) welf = self;

[self.player addPeriodicTimeObserverForInterval:CMTimeMake(10, 1000)
                                          queue:NULL // main queue
                                     usingBlock:^(CMTime time) {

  float totalSeconds = (Float64)(time.value * 1000) / (Float64)(time.timescale);
  NSLog(@"progress %f", totalSeconds);

                                                }];

logs:

App[2373:792179] duration: 3968.00

hit play button

App[2373:792179] progress 0011.176
App[2373:792179] progress 0021.175
...
App[2373:792179] progress 3701.319
App[2373:792179] progress 3704.000

Should not I expect last number to be 3968.0 ?

Audio is streamed from server.