I can access my private variables in a category but not framework variables. Why?

Question

All instance variables within the scope of the class are also within the scope of the category. That includes all instance variables declared by the class, even ones declared @private.

Therefore, I can do this (and it works):

@interface MyControl : UISearchDisplayController
{
    @private
    int privateInteger;
}
@end

@implementation MyControl
@end

@interface MyControl (Private)
@end

@implementation MyControl (Private)

- (void)myMethod
{
    privateInteger = 0;
}

@end

But why I cannot do this (I get an undefined symbol link error) :

@interface UISearchDisplayController (MyCategory)
@end

@implementation UISearchDisplayController (MyCategory)

- (void)myMethod
{
    _dimmingView = nil;
}

@end

_dimmingView is a private variable in UISearchDisplayController.
Is there a build option that allows me to do that, of which I do not know about?