Like in many games, the main map is most likely made out of individual screens that are 256 * 240 pixels in size. Since all those "garbage" screens appear to be in places not accessible to the player, I guess that those screens are not actively "set" in memory, so the screen loaded when they are visited through a cheat simply corresponds to whatever value is in memory. Reminds me of what happened when learning C++ on college when we would access a variable whose value had not been initialized beforehand; we'd get whatever value was in that memory location which could be anything.
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Current projects: Ristar-The Shooting Star (Gen), Sonic The Hedgehog (Gen), Sonic CD (Sega CD), Mega Man Zero (GBA), Battletoads (NES), Bucky O'Hare (NES)