>>58185371It's cause of the way shiny Pokemon are generated. A Pokemon's PID, which links to its IVs, Nature, and Ability, is determined when you first see it in battle. However its shininess is determined by the original NPC trainer's TID and SID. The shiny calculation is redone when you snag it. Which means it's possible for an NPC to send out a shiny Shadow Pokemon, and then after you snag it it stops being shiny. Likewise they can send out a non-shiny Pokemon, and after you snag it it turns shiny.
Colosseum is SUPPOSED to have a check where if the player's TID and SID would render that Pokemon shiny the PID advances to the next frame to prevent it, but due to a glitch it doesn't work. However, it works properly with Ho-Oh. The glitch was fixed in Gale of Darkness. So as a rule only Shadow Pokemon in Colosseum can be shiny, while only non-Shadow Pokemon in Gale of Darkness can be shiny.
If you want the National Ribbon on a shiny Pokemon it's gotta come from Colo.
