Sorting an NSMutableArray with custom objects 'overwrites' some objects

For a little iPhone application I am making, I want to sort a NSMutableArray.

I found 2 ways of doing this, but they both result in the same thing. Sorting the array will cause some objects to 'overwrite' eachother.

First off, here is my code:

AppDelegate.h

NSMutableArray* highScores;

Somewhere down that AppDelegate.h, I also make this variable a property so that I can access it from differen classes:

@property (retain, nonatomic) NSMutableArray* highScores;

When my application starts, I read the high scores from a file and import them into my NSMutableArray.

AppDelegate.m

NSMutableData* data = [NSData dataWithContentsOfFile:highScoresPath];
NSKeyedUnarchiver* decoder = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
self.highScores = [decoder decodeObjectForKey:@"highscoresArray"];

The objects I store in this NSMutableArray are from the type HighScore.

HighScore.h

@interface HighScore : NSObject {

    int score;
    int roundsPlayed;
    int wrongAnswers;

    NSString* name;

    NSDate* datetime;
}

@property int score;
@property int roundsPlayed;
@property int wrongAnswers;
@property (nonatomic, copy) NSDate* datetime;

@property (nonatomic, copy) NSString* name;

- (id) init;
- (void) update:(int)roundScore:(BOOL) correct;

@end

HighScore.m

#import "HighScore.h"

@implementation HighScore

@synthesize score, roundsPlayed, wrongAnswers, name, datetime;

- (id) init
{
    self.name = @"";
    self.score = 0;
    self.roundsPlayed = 0;
    self.wrongAnswers = 0;

    self.datetime = [NSDate date];

    return self;
}

- (void) update:(int)roundScore:(BOOL) correct
{
    self.score += roundScore;

    if (!correct)
        self.wrongAnswers++;

    self.roundsPlayed++;
    self.datetime = [NSDate date];
}

- (id) initWithCoder:(NSCoder *) decoder 
{   
    self.name = [[decoder decodeObjectForKey:@"name"] retain];
    self.score = [decoder decodeIntForKey:@"score"];
    self.roundsPlayed = [decoder decodeIntForKey:@"roundsPlayed"];
    self.wrongAnswers = [decoder decodeIntForKey:@"wrongAnswers"];
    self.datetime = [[decoder decodeObjectForKey:@"datetime"] retain];

    return self;
}

- (void) encodeWithCoder:(NSCoder *)encoder 
{
    [encoder encodeObject:self.name forKey:@"name"];
    [encoder encodeInt:self.score forKey:@"score"];
    [encoder encodeInt:self.roundsPlayed forKey:@"roundsPlayed"];
    [encoder encodeInt:self.wrongAnswers forKey:@"wrongAnswers"];
    [encoder encodeObject:self.datetime forKey:@"datetime"];
}

- (NSComparisonResult) compareHighscore:(HighScore*) h
{

    return [[NSNumber numberWithInt:self.score] compare:[NSNumber numberWithInt:h.score]];
}

@end

Now, when I try to sort my array by using the following code:

NSArray *sortedArray;
sortedArray = [highScores sortedArrayUsingSelector:@selector(compareHighscore:)];

It somehow screws up my highScores array, I get an X amound of highscores with the same score and name.

What am I doing wrong?