Wibbly-wobbly timey-wimey stuff.
New Who's "chronology" has never been consistent. The RTD era was allegedly taking place one chronological year after broadcast from every episode commencing with "Aliens Of London" (where the Doctor lands Rose back at her estate, but a year later), but you'd be hard pressed to actually use that as a basis for a proper timeline, and there's just as much evidence to contradict it as there is to support it (even more so once Torchwood
meanders into the picture as well).
Unless an explanation is given on-screen, it's best just to ignore it.