Differentiate between similar images

I have take a screen shot of a image.
This image is common on screen, now Sikuli is identifying many images.
I have already tried

reg.exists(xxx.img,100)
reg.click(reg.getLastMatch())

But the problem is that the number of this particular image changes,lets say initially if its 10.
Next time when I will run my script number of image could be 12 or 15
I had already set the maximum match to 10/10

In this 10th match is the desired image which I want

Now something very strange is happening.

reg.getLastMatch() is not selecting the image which I want...

What sikuli is doing, the image which I want is not in those 10/10 images it has become 11th match now.

what should I do???