This was one of the main points that people brought up to disprove John Titor, but it also strengthened his case. You see, according to him, he had come back in time to acquire an IBN 5100, and gave many predictions about what would happen in the future. Some of these came true, though they were very vague, so it's not much of a stretch for them to have been predicted, and some didn't. People thought this disproved his idea of time traveler, and he must have been lying, but he stood by his claims, especially about those of alternate time lines. This meant that all of his predictions would have been correct in his time line, but not ours, because he had changed the past by coming to our time line, therefore creating an alternate time line.Basically this means that, by coming to warn us about the future, the time travelers have already begun moving our time line away from their own. The choices we make then change our future further. However, by coming to their past, they can influence the future. Say, for example, I came to the year 1935 and warned everyone of the oncoming WWII, and the threat posed by Hitler and the Nazi party. This could influence the Nazi party and Hitler to rise to power, or even give Hitler the very idea for the Nazi party, and, therefore, all I have done is ensure that WWII happens, instead of actually changing anything.
Cave Ab Veritas.