On a 32-bit iPad rounding is correct, However, on a 64-bit iPad the rounding errors occurs. Since rotation by M_PI is the same as scaling by -1, this can be easily fixed by changing it to:
CGAffineTransform preferredTransform = CGAffineTransformMake(-1, 0, 0, -1, 1920, 1080);
CGAffineTransform transform1 = CGAffineTransformConcat(CGAffineTransformMakeTranslation(-preferredTransform.tx, -preferredTransform.ty), **CGAffineTransformMakeScale(-1, -1)**);
NSLog(@"%@", NSStringFromCGAffineTransform(transform1));