Inaccurate accelerometer update interval - iphone

Question

I'm using the CoreMotion API and I would like to save the accelerometer's values every 10ms (100hz). The update intervals I obtained so far aren't accurate.
Here's an example of update intervals I got (in second):
0.010414999997
0.0105919999769
0.0117060000193
0.0198359999922
0.00989700001082
0.0100809999858
0.0100519999978
0.0106810000143
0.010420000006
0.0107459999854
0.0105899999908
0.0105130000156
0.0104829999909
0.0107439999992
0.010391000018
0.0105859999894
0.0102320000005
0.010134000011
0.0101929999946
0.010666999995
0.00996399999713
0.0123709999898
0.0181950000115
0.0107940000016
0.00988500000676
0.0101469999936
0.0103529999906
As you can see, some values are higher than 10ms.
Some more informations:
- Xcode 3.2.6 / iOS4.3 / armv7
- Tested on iPhone4 iOS5.1 and iPodTouch4 iOS4.3.3
- The source code:
-(void) viewDidLoad {
started = NO;
motionManager = [[CMMotionManager alloc] init];
if ([motionManager isAccelerometerAvailable]) {
motionManager.accelerometerUpdateInterval = 0.01; //100Hz
motionQueue = [[NSOperationQueue alloc] init];
[motionQueue setMaxConcurrentOperationCount:1]; // Serial operation queue
} else {
NSLog(#"Accelerometer is not available!\n");
}
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSMutableString *documentsDirectory = [[NSMutableString alloc] initWithString:[paths objectAtIndex:0]];
path = [[NSMutableString alloc] init];
[path setString:[NSString stringWithFormat:#"%#/timestamp.cap", documentsDirectory]];
NSFileManager *fileManager = [NSFileManager defaultManager];
[fileManager removeItemAtPath:path error:NULL];
[fileManager createFileAtPath:path contents:nil attributes:nil];
}
- (IBAction) startStopButton:(id) sender {
if(!started) {
started = YES;
[sender setTitle:#"Stop" forState:UIControlStateNormal];
CMAccelerometerHandler dataHandler = ^(CMAccelerometerData *accelerometerData, NSError *error) {
NSString *content = [NSString stringWithFormat:#"%f\n", accelerometerData.timestamp];
NSFileHandle *fh = [NSFileHandle fileHandleForWritingAtPath:path];
[fh seekToEndOfFile];
[fh writeData:[content dataUsingEncoding:NSUTF8StringEncoding]];
[fh closeFile];
};
[motionManager startAccelerometerUpdatesToQueue:motionQueue withHandler:dataHandler];
} else {
started = NO;
[motionManager stopAccelerometerUpdates];
[motionQueue waitUntilAllOperationsAreFinished];
[sender setTitle:#"Start" forState:UIControlStateNormal];
}
}
Thank you in advance for your answers.