What does CADisplayLink's timestamp property mean?

Hardware: iPhone 6

Program:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  ...
  globalDisplayLink = [[myWindow screen] displayLinkWithTarget:self selector:@selector(doFrame:)];
  [globalDisplayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}

- (void)doFrame:(CADisplayLink *)displayLink
{
  static u32 frameNum = 0;

  static CFTimeInterval tPrev = (CFTimeInterval)0.0f;
  CFTimeInterval tNow = [displayLink timestamp];
  f32 dt = (f32)(tNow - tPrev);
  tPrev = tNow;

  static u64 machTPrev = 0;
  u64 machTNow = mach_absolute_time();
  u64 machDt = machTNow - machTPrev;
  machTPrev = machTNow;
  f32 machDtF32 = machTimeToSeconds(machDt);

  if(frameNum++ > 0)
  {
      NSLog(@"(%d)\tDisplayLink: %f\tMach: %f", frameNum, dt, machDtF32);
      if(frameNum == 3)
      {
            NSLog(@"sleeping..");
            [NSThread sleepForTimeInterval:WAIT_INTERVAL];
      }
  }
  ...
  [myEAGLContext presentRenderbuffer:GL_RENDERBUFFER];
}

Output (WAIT_INTERVAL = 5.0f):

(2) DisplayLink: 0.016675 Mach: 0.010766

(3) DisplayLink: 0.016674 Mach: 0.016691

sleeping..

(4) DisplayLink: 0.266828 Mach: 5.012117

(5) DisplayLink: 4.752381 Mach: 0.007276

(6) DisplayLink: 0.016714 Mach: 0.018646

Output (WAIT_INTERVAL = 0.02f):

(2) DisplayLink: 0.016700 Mach: 0.016742

(3) DisplayLink: 0.016662 Mach: 0.016679

sleeping..

(4) DisplayLink: 0.016658 Mach: 0.022046

(5) DisplayLink: 0.016705 Mach: 0.011410

Discussion:

The docs say about timestamp: "The time value associated with the last frame that was displayed". Additionally, it reads "The target can read the display link’s

timestamp

• WAIT_INTERVAL == 5.0f: Not sure how or why the 5s timestep was broken up into 0.26s + 4.75s over 2 frames. There is no way this can represent the time when the last frame was shown.
• WAIT_INTERVAL == 0.02f: How does this not even register?
• Overall (without the sleeping), [CADisplayLink duration] seems to cover much closer to 16.6ms, where dt computated via mach_absolute_time() has much more variance, regularilly dipping close to 14ms, and going above 20ms. I thought mach_absolute_time() was highest res clock available on iOS?