When learning about the Mexican Revolution in history class, we had to learn about all of the perspectives of historians and their outlooks on the subject. Why are multiple inputs from historians needed if there is only one history that happened? Why do we need to learn about these interpretations when there is only one way that the history worked?
"History" as a body of knowledge and a field of study is not synonymous with "the past." There might be only one past that happened, but our knowledge of it is not absolute.
It's a bit dated now, but E.H. Carr's classic What is History? still covers this relatively well in my opinion. The first chapter ("The Historian and His Facts") is essentially a critique of the "common sense" notion of history, which is that
History consists of a corpus of ascertained facts. The facts are available to the historian in documents, inscriptions, and so on, like fish on the fishmonger's slab. The historian collects them, takes them home, and cooks and serves them in whatever style appeals to him.
This is a particular conception of history that was popular among 19th and many early 20th century historians, where the goal of history was to tell the story of the past "as it really was" (wie es eingentlich gewesen in words of the 19th century German historian Leopold von Ranke; this type of history is sometimes called "Rankean" in reference to him). To the historians and philosophers who subscribed to this view, history was an empirical, objective process of gathering facts to recreate "what really happened." Although most historians no longer believe in such a notion, this conception of history has profoundly influenced how many non-historians see the field.
However, as Carr points out, this is an very naive view of how history works. To condense his argument a bit, Carr effectively makes three charges against the "common-sense" or "Rankean" view of history:
Facts are made by the historian. The Rankean view relies on "a complete separation between subject and object," or in other words that the researcher is separate from the thing she is researching. However, historians do not quite work with a pre-existing body of facts. Historians decide subjectively what is relevant and what is not when researching or writing history. Carr makes a distinction between "historical facts" (i.e., facts that are widely accepted to have relevance and be part of "history" as such) and "facts of the past" (which are facts that are deemed to be non-relevant and rarely, if ever, incorporated into formal histories). Thus, the historian selects the facts on which history is based, a process which is always at least partially subjective and informed by professional, sociological, and yes, political assumptions.
Historical artefacts are biased. Carr also notes that even if historians were capable of getting "all the facts," the problem is that "the facts" do not quite speak for themselves. He notes that a great deal of what we know about ancient Greece is in fact written by Athenians — and predominantly older, educated, free male Athenians to boot (he wrote this in 1961, so I'm not sure how true this is today). Consequently, "Our picture [of Greece] has been pre-selected and predetermined for us, not so much by accident as by people who were consciously or unconsciously imbued with a particular view and thought the facts which supported that view worth preserving." The authors of historical documents are just as selective and imperfect as historians themselves, because they too operate with a limited knowledge of their own time and had their own social and ideological interests, bringing us even further away from "what really happened."
History is based on forgetting as well as remembering. This isn't really a point Carr makes explicitly, but it's the necessary corollary to the second point. The vast, vast majority of people who have ever lived have lost their voices to history. These voices tend to be from marginalized or oppressed people, because until relatively recently the few who could afford the requisite education to read and write would be from privileged sections of society. Because most people don't leave behind written accounts of their lives, we have to read what sources we do have creatively in order to capture what it might have been like for them. This opens up even more room for conflicting interpretations and methods.
Subjectivity is thus inextricably tied to history. Knowledge of the past (history) is always partial and filtered through historians' methodological assumptions, which are in turn influenced by a wide variety of considerations. This means that different historians will draw slightly (or significantly) different pictures of the past. That is why you are exposed to a multiplicity of viewpoints from numerous different scholars.
Readings
Historiography is a big field (and sort of intimately linked with philosophy of history), but a few things stand out. Geoffrey Elton wrote The Practice of History as a critical response to Carr. Richard J. Evans' In Defense of History is a more recent work that was mostly well-received by historians (as far as I know). Peter Novick's That Noble Dream: The 'Objectivity Question' and the American Historical Profession is a sort of meta-history of how historians have approached this question. Hayden White's Metahistory is a post-modern take on things, and goes even further by suggesting that history is actually a kind of literature.