How does similarity work?

I am updating some images for some tests and noticed that sikuli acts a little differently with some of the images I'm using.

For instances:
1. I had a Modify button (81x31) that was captured, but when retested the Hide button was found. Similarity needed to be increased.
2. I had a language indicator image (140x11) that could not be found when retested. Similarity needed to be decreased.

I understand the principle of less background and focus on differentiators, plus use regions. These don't seem to be helping though.
When I looked at the two images of instance 2, they seemed pixel for pixel the same, but Sikuli saw the difference between the two.

I read the article on the OpenCV matchTemplate, but that's not similarity, is it?