An unsigned editorial is meant to represent the opinion of the newspaper.
Many larger daily newspapers, the New York Times included, have editorial boards consisting of several people whose job it is to find consensus on topics about which the paper takes a stand. One of the editorial board members is assigned the task of writing the editorial.
At many smaller papers, the "editorial board" may consist on one person -- often the publisher or the editorial page editor.
At the weekly paper I once edited, I wrote all the editorials. At one point I had another staffer who handled the editorial page, and then we took turns writing editorials. In the interest of transparency, we began signing the editorials with our initials. Some readers really liked that we did it that way, but it took away the ability to editorialize anonymously in situations where it probably would have been best to do so.
It's a matter of preference by the paper in question.
