NavigationController和scrollView的问题

我把DiscoverController放在一个NavigationController里面然后再添加到tabbarcontroller里 然后我定义了一个scrollview 它的frame是 (0 0 self.view.frame.size.width, 108) (这里设置108是因为20是状态栏＋44导航栏＋44是我希望可以显示的scrollview的高度) 然后我添加一个UIView到scrollview上 这个uiview的frame是(100,0,500, 44). 我希望达到的效果是这个uiview只能左右滚动不能上下滚动所以我还设置了scrollview的contentsize是(900, 0). 但是很奇怪我添加了这个uiview之后还是能上下滚动 可是当我把scrollview的Y值设置为－5或者scrollview的height设置为49， 就变得只能左右滚动不能上下滚动 我想知道这个5是来自哪里的 为什么要把Y设置成－5才能达到只能左右不能上下滚动? 一般设置scrollview的滚动不是用contentsize 来决定的吗??

问题可能我没有说的很清楚  反正就是有5个pixel我不知道是从哪里来的 想求各位大神指教
代码如下：

#import "DiscoverController.h"
#import "DiscoverNaviButton.h"
#import "LocationController.h"

@interface DiscoverController () <UITableViewDataSource, UITableViewDelegate, LocationControllerDelegate>
@property(nonatomic, strong) LocationController *locationList;
@property(nonatomic, strong) DiscoverNaviButton *btn;
@property(nonatomic, weak) UIScrollView *typeScrollView;
@end

@implementation DiscoverController

- (LocationController *)locationList{
    if(!_locationList){
        _locationList = [[LocationController alloc] initWithStyle:UITableViewStylePlain];
        UIButton *rightBtn = [UIButton buttonWithType:UIButtonTypeCustom];
        [rightBtn setImage:[UIImage imageNamed:@"rightBtn"] forState:UIControlStateNormal];
        rightBtn.bounds = CGRectMake(0, 0, 20, 20);
        [rightBtn addTarget:self action:@selector(dismissVC) forControlEvents:UIControlEventTouchUpInside];
        _locationList.delegate = self;
        UIBarButtonItem *rightBarBtn = [[UIBarButtonItem alloc] initWithCustomView:rightBtn];
        _locationList.navigationItem.rightBarButtonItem = rightBarBtn;
        _locationList.location = @"Toronto";
    }
    return _locationList;
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor blackColor];
    
    [self setupScrollViewBtn];
    [self setupButtonTitle];
}
- (void)setupScrollViewBtn{
    CGFloat scrollViewHeight = self.navigationController.navigationBar.frame.origin.y + self.navigationController.navigationBar.frame.size.height + 49;
    UIScrollView *typeScrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, -5, self.view.frame.size.width, scrollViewHeight)];
    typeScrollView.showsHorizontalScrollIndicator = NO;
    typeScrollView.backgroundColor = [UIColor blueColor];
    typeScrollView.contentSize = CGSizeMake(600, 0);
    
    
    UIView *testView = [[UIView alloc] initWithFrame:CGRectMake(100, 5, 400, 44)];
    testView.backgroundColor = [UIColor yellowColor];
    [typeScrollView addSubview:testView];
    [self.view addSubview:typeScrollView];
}
- (void)setupButtonTitle{
    DiscoverNaviButton *button = [[DiscoverNaviButton alloc] init];
    button.frame = CGRectMake(0, 0, 80, 20);
    [button setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    [button setTitle:self.locationList.location forState:UIControlStateNormal];
    [button setImage:[UIImage imageNamed:@"naviBtnArrow"] forState:UIControlStateNormal];
    [button addTarget:self action:@selector(presentVC) forControlEvents:UIControlEventTouchUpInside];
    self.btn = button;
    self.navigationItem.titleView = self.btn;
}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
- (void)presentVC{
    UINavigationController *naviLocation = [[UINavigationController alloc] initWithRootViewController:self.locationList];
    [self presentViewController:naviLocation animated:YES completion:nil];
}
- (void)dismissVC{
    [self.locationList dismissViewControllerAnimated:YES completion:nil];
    self.locationList = nil;
}
- (void)locationController:(LocationController *)locationController didSelectLocation:(NSString *)location{
    [self setupButtonTitle];
    [self.locationList dismissViewControllerAnimated:YES completion:nil];
    self.locationList = nil;
}

- (void)changeType:(UIButton *)btn{
    btn.selected = !btn.isSelected;
}
@end