When placed on a flat surface, light can be seen underneath the center of the level. The slight curvature is an important part of the process of creating a properly functioning wood level.
The curvature is known as "hollow grind," and is created in the manufacturing process to ensure that when the level is placed on a masonry surface, it will rest on its ends and not rock back and forth in the middle, as it might do if the edge of the level were perfectly flat. This results in a more accurate reading.
Another reason for the hollow grind is in the nature of the wood itself. Over time, wood will gain or lose moisture content resulting in small dimensional changes. By creating a slight curvature, we are able to compensate for these changes - in fact, Stanley chose mahogany for our levels because of its exceptional dimensional stability over a variety of temperature and humidity conditions.
Be assured that by including this extra step in our manufacturing, we are producing a better product.