I am facing a strange problem with my iphone. It shows available memory as 278 Mb from settings and also in the itunes . But when i find it programatically like this

NSDictionary *fileSystemAttributes = [[NSFileManager defaultManager] attributesOfFileSystemForPath:NSHomeDirectory() error:&error];
double availableSpace = [[fileSystemAttributes objectForKey:NSFileSystemFreeSize] floatValue];

I am getting it as 458.0 Mb. Can any one help me out why i m having so much difference between both the values ?? As both the values should be same

Thanks in advance