I am reading this news article about the Rosetta mission.
In one paragraph it says:
Philae's data collection from a comet travelling at 18 kilometres (11 miles) per second, currently at a distance of 510 million kilometres (320 million miles) from Earth, crowns a 10-year mission to study the origins of Solar System some 4.6 billion years ago, and maybe even life on Earth.
This means that the comet is about 510 million kilometers distant.
But in the another paragraph it says like this:
Philae landed Wednesday after a nail-biting seven-hour, 20-km descent from Rosetta, which had travelled more than a decade and 6.5 billion kilometres (four billion miles) to meet up with comet 67P/Churyumov-Gerasimenko in August this year
So my question is why did Rosetta traveled this long distance of 6.5 billion kilometers?