Any of a number of reasons. It could be that older estimates are being used instead of newer orbital estimates as we understand its orbit better. It could also be just plain misunderstanding of the technical information in the popular press. You will have to visit authoritative sites such as the NASA/JPL comet home page, Sky and Telescope and Astronomy magazines to get the most accurate information provided by Dr. Brian Marsden at the Smithsonian Astrophysical Observatory in Cambridge. He is the world expert on minor body orbit calculations.