Simple leak not showing in Instruments-Leak tool

Why when the following code (intentionally leaky) is executed with the Instrument-Leaks tool, does it show that there is a leak for NSObject, but not NSDate? They both show up when run with the Analyze static analysis tool as I would expect.

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {

    NSObject* obj = [NSObj开发者_StackOverflowect alloc];
    obj = [NSObject alloc];

    NSDate* date = [NSDate alloc];
    date = [NSDate alloc];

    sleep(10); // time to allow leaks to pick up sample
    return 0;
}

This is just a assumption: NSDate could implement some pretty-hard hacks for performance which makes its -alloc method return a cached value.