Exposure measurement systems work by evaluating the light content in the image frame. The evaluation process does not necessarily have to look at the entire frame. It can look at just a small part of the center of the image, and ignore the outer edges. These are the two extreme cases.
Different exposure modes vary the size of the center area being evaluated, as well as, varying the amount of weight given to the center area to the outer edges of the image frame. That's a layman's explanantion of how automatic exposure metering works.
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