NSAssert in Singleton : why this code is valid?

i don't understand the use of NSAssert in +alloc, as when +alloc is called from +sharedGameManager, the static _sharedGameManager variable is nil (so NSAssert should stop execution the first time [self alloc] init] is called...)

+(GameManager*)sharedGameManager {
     @synchronized([GameManager class])                             
     {
         if(!_sharedGameManager)                                    
             [[self alloc] init]; 
         return _sharedGameManager;                                 
     }
     return nil; 
 }
 
 +(id)alloc 
 {
     @synchronized ([GameManager class])                            
     {
         NSAssert(_sharedGameManager == nil,
                  @"Attempted to allocated a second instance of the Game Manager singleton");
         _sharedGameManager = [super alloc];
         return _sharedGameManager;                                 
     }
     return nil;  
 }
 

Thanks for your answer

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