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By closing one eye, you removed binocular vision, the slight variance in images, called “binocular disparity,” that occurs when we view an object with both eyes open. Binocular vision—sometimes called “depth perception”—allows us to see the world as three-dimensional. When you close one eye, the single image is two-dimensional—that is, it is flat, like a photograph, and therefore can be “copied” onto flat paper.
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Betty Edwards (Drawing on the Right Side of the Brain: A Course in Enhancing Creativity and Artistic Confidence: The Definitive 4th Edition)