How to adjust UIToolBar left and right padding

I create one UIToolbar with code and another with interface builder. But, found out the two toolbar having different left and right padding which shown below:

From Interface Builder:

From Code:

UIImage *buttonImage = [[UIImage imageNamed:@"button.png"] stretchableImageWithLeftCapWidth:10 topCapHeight:0];
UIButton *btnTest = [UIButton buttonWithType:UIButtonTypeCustom];
[btnTest setBackgroundImage:buttonImage forState:UIControlStateNormal];
[btnTest setTitle:@"Back" forState:UIControlStateNormal];   
[btnTest.titleLabel setFont:[UIFont boldSystemFontOfSize:13]];  
[btnTest setBackgroundImage:[imgToolbarButton stretchableImageWithLeftCapWidth:5 topCapHeight:0]  forState:UIControlStateNormal];
[btnTest addTarget:self action:@selector(clearDateEdit:) forControlEvents:UIControlEventTouchUpInside];
btnTest.frame = CGRectMake(0.0, 0.0, 50, 30);
UIBarButtonItem *btnTestItem = [[UIBarButtonItem alloc] initWithCustomView:btnTest];
[self.toolbar setItems:[NSArray arrayWithObjects:btnTestItem,nil]];
[btnTestItem release];

My question is how can I adjust the left and right padding of UIToolbar by code?

Update

I discovered this alignment issue only happen to UIBarButtonItem with customView of UIButton, the alignment is fine with UIBarButtonItem. Any idea what cause this or to resolve this.

The only solution I can think of right now is to manually set the frame.