Objective-C, NSMutableData not released

My app sometimes reads data, uses it, then might need to read it again later. I noticed that if I read the data once it gets released, but if I happen to read it again it never gets released.

Why is the data not released if read again?

Code:

#import "AppDelegate.h"

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    [self readData]; // read one
    NSLog(@"read1");
    sleep(1);
    [self readData2];
}

- (void)readData2 {
    [self readData]; // read two
    NSLog(@"read2");
}

- (void)readData {
    __block NSMutableData *data = [NSMutableData dataWithContentsOfFile:@"test"]; // file is 125 MB
    NSUInteger size = [data length];
        for (NSUInteger i = 0; i < size; i++) {
            // do stuff
        }
        return;
}
@end

Try using dataWithContentsOfFile:options:error with option:

NSDataReadingMappedIfSafe

A hint indicating the file should be mapped into virtual memory, if possible and safe.

Although the description of dataWithContentsOfFile does not indicate, not using the options will likely result in data being retained otherwise.

dataWithContentsOfFile

This method is equivalent to dataWithContentsOfFile:options:error: with no options. If you need to know what was the reason for failure, use dataWithContentsOfFile:options:error:.