UIView animation and transform cause a weird position offset - objective-c

Question

http://childhoodgamedev.qiniudn.com/xincheng_testGif.gif
UILabel* label = [[UILabel alloc] initWithFrame:CGRectMake(250, 250, 200, 100)];
label.text = #"Hello world!";
label.textColor = [UIColor blackColor];
label.textAlignment = NSTextAlignmentLeft;
label.backgroundColor = [UIColor clearColor];
label.font = [UIFont systemFontOfSize:25];
[self.view addSubview:label];
CGAffineTransform transform = CGAffineTransformIdentity;
label.transform = CGAffineTransformScale(transform, 2, 2);
[UIView animateWithDuration:4.0 delay:0 options:UIViewAnimationOptionCurveLinear animations:^{
label.transform = CGAffineTransformTranslate(label.transform, -100, 0);
} completion:^(BOOL finished) {
if (finished) {
NSLog(#"label %f %f %f %f", label.frame.origin.x, label.frame.origin.y, label.frame.size.width, label.frame.size.height);
[UIView animateWithDuration:3.0 delay:0 options:UIViewAnimationOptionCurveLinear animations:^{
NSLog(#"label %f %f %f %f", label.frame.origin.x, label.frame.origin.y, label.frame.size.width, label.frame.size.height);
label.transform = CGAffineTransformTranslate(label.transform, 100, 0);
} completion:NULL];
}
}];
There is UILabel instance named label, it's scaled to 2 ratio by CGAffineTransformScale. Then it moves from right to left, when the animation is completed, then it moves from left to right.
But the strange point is: It looks not the same position, which is the end point of animation(right to left), the start point of animation(left to right). But the NSLog tells me, they are in the same position.
http://childhoodgamedev.qiniudn.com/xincheng_QQ20150306-1.png
When I comment the state label.transform = CGAffineTransformScale(transform, 2, 2);, everything is fine. When the label change direction to move, the end position of right->left animation is just the start position of left->right animation.
So I think the problem is caused by CGAffineTransformScale. Any ideas?