title: UIScrollView behave strange
body: <p>I set UIScrollView's subview's height to 768,and UIScrollView's contentSize is 1024*768 .But when I swipe up or swipe down,the subview(UIImageView) will scroll up or down a little.I don't know why.</p>
  
  <p>here is my code</p>
  
  <pre><code>    -(void)viewDidLoad
  {
      [super viewDidLoad];
  
      // scrollView is an outlet of UIScrollView
      [scrollView setBackgroundColor:[UIColor blackColor]];
      [scrollView setCanCancelContentTouches:NO];
      scrollView.indicatorStyle = UIScrollViewIndicatorStyleWhite;
      scrollView.clipsToBounds = YES;     // default is NO, we want to restrict drawing within our scrollview
      scrollView.scrollEnabled = YES;
  
      // pagingEnabled property default is NO, if set the scroller will stop or snap at each photo
      // if you want free-flowing scroll, don't set this property.
      scrollView.pagingEnabled = YES;
  
      // load all the images from our bundle and add them to the scroll view
      NSUInteger i;
      imageCount=3;
      for (i = 1; i &lt;= imageCount; i++)
      {
          NSString *imageName = [NSString stringWithFormat:@"tu%d_s.png", i];
          UIImage *image = [UIImage imageNamed:imageName];
          if(!image)
          {
              NSLog(@"%@ is nil",imageName);
          }
          UIImageView *TempImageView = [[UIImageView alloc] initWithImage:image];
  
          // setup each frame to a default height and width, it will be properly placed when we call "updateScrollList"
          CGRect rect = TempImageView.frame;
          rect.size.height = 768;
          rect.size.width = 1024;
          TempImageView.frame = rect;
          TempImageView.tag = i;  // tag our images for later use when we place them in serial fashion
          [scrollView addSubview:TempImageView];
          NSLog(@"viewDidLoad TempImageView width %f,height %f,x %f,y %f",TempImageView.frame.size.width,TempImageView.frame.size.height,TempImageView.frame.origin.x,TempImageView.frame.origin.y);
  
      }
  
      [self layoutScrollImages];  // now place the photos in serial layout within the scrollview
  
  }
  
  - (void)layoutScrollImages
  {
      UIImageView *view = nil;
      NSArray *subviews = [scrollView subviews];
  
      // reposition all image subviews in a horizontal serial fashion
      CGFloat curXLoc = 0;
      for (view in subviews)
      {
          if ([view isKindOfClass:[UIImageView class]] &amp;&amp; view.tag &gt; 0)
          {
              CGRect frame = view.frame;
              frame.origin = CGPointMake(curXLoc, 0);
              view.frame = frame;
  
              curXLoc += (1024);
          }
          NSLog(@"shang2 layoutScrollImages width %f,height %f,x %f,y %f",view.frame.size.width,view.frame.size.height,view.frame.origin.x,view.frame.origin.y);
      }
  
      // set the content size so it can be scrollable
      [scrollView setContentSize:CGSizeMake((imageCount * 1024), 768)];
      NSLog(@"shang2 content width %f,height %f,x %f,y %f",scrollView.contentSize.width,scrollView.contentSize.height,scrollView.frame.origin.x,scrollView.frame.origin.y);
  }
  </code></pre>
  
  <p>here is Xcode log</p>
  
  <pre><code>2013-04-03 20:50:23.939 animation[1464:c07] viewDidLoad TempImageView width 1024.000000,height 768.000000,x 0.000000,y 0.000000
  2013-04-03 20:50:23.952 animation[1464:c07] viewDidLoad TempImageView width 1024.000000,height 768.000000,x 0.000000,y 0.000000
  2013-04-03 20:50:23.995 animation[1464:c07] viewDidLoad TempImageView width 1024.000000,height 768.000000,x 0.000000,y 0.000000
  2013-04-03 20:50:23.995 animation[1464:c07] shang2 layoutScrollImages width 7.000000,height 5.000000,x 741.000000,y 1019.000000
  2013-04-03 20:50:23.996 animation[1464:c07] shang2 layoutScrollImages width 1024.000000,height 768.000000,x 0.000000,y 0.000000
  2013-04-03 20:50:23.996 animation[1464:c07] shang2 layoutScrollImages width 1024.000000,height 768.000000,x 1024.000000,y 0.000000
  2013-04-03 20:50:23.996 animation[1464:c07] shang2 layoutScrollImages width 1024.000000,height 768.000000,x 2048.000000,y 0.000000
  2013-04-03 20:50:23.996 animation[1464:c07] shang2 content width 3072.000000,height 768.000000,x 0.000000,y 0.000000
  </code></pre>