Why are some Pokedexes different than other in different regions if they are the same invention?
Because thats the model they came in, sometimes adding new features to the pokedex. All Cell Phones are basically the same but they come in all different shapes and forms.
Well, it's kind of like technology today. Every time you beat the Elite 4, your pokedex gets up-graded to the National Dex, which is basically the up-to-date version, meaning that the data for the pokedex numbers may change.