Often they are, and it's actually not an error. This is caused by the fact that Earth moves around the sun more quickly than either Voyager spacecraft is departing from Earth. read more
http: //voyager.jpl.nasa.gov/where/ The answer is right there on the page: Note: Because Earth moves around the sun faster than Voyager 1 is traveling from Earth, the distance between Earth and the spacecraft actually decreases at certain times of the year. read more
If the distance between the earth and the star AC +79 3888 was scaled to the distance separating New York City and Vancouver, British Columbia (3000 miles/4800 km), then Voyager 1 has covered 1/3 of a mile (590 m) of its impossibly long journey. read more