Originally posted by Foxglovez:

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If some of your coverage is from above ground, like on a second or third floor of a building, the coverabe is like a bubble, so it will be smaller area as it gets lower (if that makes sense - think of the item as the center of a ball, not just within a simple circle, the diameter changes both above and below).
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In other words real coverage is a sphere, but the circle always shows like a cylinder. Think of a sphere in a cylinder, the area in the cylinder not touched by the sphere is not covered - even though that's what the circle shows.