When a Pokémon is transferred, its location becomes simply the name of
the region it was caught rather than the exact location it was caught.
A Pokémon caught in HeartGold or SoulSilver will have its location
listed as Johto, regardless of if it was caught in Kanto, the Sinjoh
Ruins, or the Pokéwalker. A Pokémon originally caught in a Generation
III game will have the text "after a long travel through time" added
to the end of its location. In addition, the level obtained at is
changed to the level arrived at, and the date obtained becomes the
date it was transferred to the Generation V game.
SS and HG are Johto based games as they originated from Gold and Silver which only had Johto as the region.
Source: Experiance and Bulbapedia