Single post in PokéDex errors

That did not answer my question... Actually, it only makes me think the Pokedex is a little inconsistent. Porygon makes even less sense now. Here's what I found out by keeping looking. Pikachu and Raichu debut in Kanto while its preevolution Pichu comes in Johto. The line is shown in the Kanto Dex, implying that placement is based on the Pokemon's first debut. This goes for Jigglypuff and Marill's baby forms as well as others. Magmar debuts in Kanto, with Magby debuting in Johto, and Magmortar debuting in Sinnoh. The line is shown in the Johto Dex, implying that placement is based on their first form. This is also true with Tyrogue's line and Elekid's line. Porygon debuts in Kanto, Porygon2 debuts in Johto, and PorygonZ debuts in Sinnoh. The line is shown in the Sinnoh Dex, which implies the line's placement is based on the final evolution. No other Pokemon is like this that I know of. So... what's the rule? Especially with Porygon, who doesn't seem to follow anything reasonable like Pikachu or Magmar. Why are we putting Porygon in Sinnoh as opposed to Kanto, where it first came from?