succesive onPreviewFrame byte[]

I am capturing the onPreviewFrame in a byte[] and then saving this to compare with the next captured frame. Although the camera is pointed to an area with no change happening and I can see on the display that there is no change succesive frames element by elemnet are different. Any suggestion why this should be so? Also I only want the grayscale and I understand that the first height * width bytes are grayscale data, is this correct>

Statistically, there cannot be an exact match. Check out Image comparison - fast algorithm for a way of comparing images.