Different agencies work on different aspects of the business, briefs get interpreted differently, people and agencies and clients have agendas (what do they want on their expense). All of this is at the expense of the person that matters most to the brand. The consumer.
As I have written about before, the easier it is to understand a brand, the more likely you will like that brand and more importantly remember the brand/product when it comes to that all important purchase decision. If a brand is constantly shouting at consumers with 4 or 5 different messages, each with a separate tone of voice and a different look and feel this is confusing and it doesn't work.