Brand valuation: art or science?

In an article for BrandChannel ("A How-To Guide to Assessing Brand Value"), Canadian evaluator Catherine Tremblay describes the cost approach, the income approach and the market approach, then concludes:

"In a nutshell, the valuation of a brand can be more of an art than a science—but it’s an exercise that can help management identify and develop the value drivers behind the brand. Professionals, such as Chartered Business Valuators and members of the American Society of Appraisers, can provide insight and assistance in the valuation of brands and other intangible assets".

There seems to be an inherent contradiction here: if it's an art, then why should we be concerned to adopt a scientific approach -- and if it's a science, why is there such a gulf between results of the application of competing methodologies? Comments, please!