Something that I've come across in a Pokemon thread is that, apparently, in the anime the entries were written by a professor who specifically wrote bull for the Pokemon he didn't know enough about.
Westwood V "He is esteemed for his work on the
Pokédex, which he wrote the entries for. Professor Westwood admitted to Ash later in the episode, though, that he made up some of the information in the Pokédex. Westwood doesn't travel much, and merely relies on reports for his Pokédex data." From bulbapedia.
EDIT: there's also this from the same source, "In the
dub, Westwood is stated to have outright programmed the
Pokédex himself, as opposed to just writing its entries."