NSString 引用计数 负1

**_NSString *str = [[NSString alloc]init];
NSString *str7 = [[NSString alloc]initWithString:@"aaa"] //retainCount = -1
NSString *str8 = [[NSString alloc]initWithFormat:@"aaa"]; //retainCount = -1
NSString *str9 = [[NSString alloc]initWithFormat:@"a%s","aa"];//retainCount = -1
NSString *str4 = [NSString stringWithFormat:@"aaa"]; // -1

NSString str10 = [[NSString alloc]initWithFormat:@"啊"]; //retainCount = 1
NSlog(@"%d",str);
为什么中文的时候计数为1 英文时 alloc开辟空间为什么不计数