>>27037765He's talking about a real world example: If you live in the UK, you aren't going to see any Giraffes. It isn't like they don't exist, but you don't count them as part of the immediate biome.
With Pokemon, it's like that. With the anime there were primarily ~150 Pokemon (and more to see) in Kanto that are studied and examined, then they just tack on more as already having existed, elsewhere.
To explain it, it becomes a surprise for Ash and certain characters to have never seen it due to new additions being foreign, or they explain it as just being a new challenge. In world, they were always there with only a few actually being "newly discovered".
