But data for things that aren't pokemon is what's after the pokemon data. Not looking at the code, I couldn't tell you what that data is, but it's not pokemon data. So the game looks for pokemon data well past where it actually exists, and as a result gets data that is for all intents and purposes completely random. Predictable, but useless data.
So they're not "placeholders," exactly, they're just... There. It's like if you had a book that was so many pages long, and you tried to see what was in the book a few pages more than that in. That's part of the hardcover, which isn't paper and doesn't have text. But if you could somehow look at that part... Say, you cut open the cover... You'd see all the not-text that was there. And then Pokemon Blue takes a look at it, says "that looks about right," and declares that not-text to in fact be text and reads it, declares it a pokemon, and shoves it on to your screen. And then your game boy (/ emulator) responds with a "What the hell did you just put in me" and spazzes out.
