ContentOffSet incorrect when use UIScrollView

[wsof401]

It has strange display in a scrollView(Calls self.ss).
I set a UIScrollView on a UIViewController.view。
I set the scrollView the constraints that no space to the vc.view of each sides

[self.ss setParallaxHeaderView:self.header 
                                     mode:VGParallaxHeaderModeTopFill 
                                     height:300];

[self.ss setContentSize:CGSizeMake(self.view.bounds.size.width, 3000)];

self.header is a instance of UIView. It has a UIScrollView On it.When I built the project. Self.ss.contentOffSet was {-600,0}
Everything is ok if self.ss is UITableView

[stoprocent]

There is a observer there to watch for contentOffset changes to calculate new constraints. I will have a look next week. I've been super busy with regular work.