The Wizards/Time Travelers have a few theories regarding the nature of time travel. The most common is predestination or a fixed timeline. If you are familiar with the TV Show Lost, they refer to it as "Whatever happened, happened". This is partly true. When traveling relative to yourself or other time travelers, then it is a fixed timeline (in most cases). The environment you travel within sometimes is not, however. Many have demonstrated that making changes to the past does not affect the future, so they believe the file manipulates them into going to periods already setup by the file, outside of the main timeline, in order to have a playground.