Basically, a domain's value is the price someone is willing to pay...and often, that means that many will pay a common price for the domain, but one or two individuals or groups may be willing to pay significantly more, because of the unique value it holds for them. It could be something as simple as a business model that matches the domain name, making it worth $1 million to that person, or just $200 for someone without a specific use/intention for the domain.
Rick Schwarz has held domains for sale for years before finding a buyer...because what Rick thought the domain was worth was not what most people thought it was worth...but then someone came along who saw the value and bought it.