Roaming IV glitch
The roaming IV glitch is a glitch in Pokémon Ruby, Sapphire, FireRed and LeafGreen that causes roaming Pokémon to have extremely low IVs. The glitch is not present in Pokémon Emerald.
In Generation III, the data for a Pokémon's IVs are stored in a 32-bit integer. However, when a roaming Pokémon is captured, only the last eight bits are transferred. While the HP IV is unaffected, the Pokémon can only have a maximum Attack IV of 7. The other IVs will always be 0.
It appears to be a glitch that is in every R/S game, since it's related to the how the data cards' storage system works.
It seems that the main Pokemon suffering from it are Latios and Latias, so if you want these two, Emerald would oblige much better than R/S.