Apache Solr Search Patterns

What does the clause DisjunctionMaxQuery((text:ipod | name:ipod^2.0)) do? Here we search for ipod in the fields text and name and find the score in both the cases, that is the scores for ipod in text and ipod in name, and return the maximum of these two scores as the output of the DisjunctionMaxQuery clause. Why do we return the maximum score? Why not take the sum and return that as the score? Suppose, we are searching for usb charger in two fields, name and description. If we take the sum, then a document that has usb charger in the name and not in the description will have the same score as another document that has usb in both name and description but charger nowhere in these fields.