>>47065138That's just because Psychic, Ghost, Dark, and Fairy can all be considered to be non-literal types.
There's no argument about whether something should be Fire or Grass or Ice type because those are all extremely literal and obvious. Things that use fire, heat, and lava are Fire-type. Things that either are made of or use plants are Grass-type. Things that use ice, snow, and the cold are Ice-type.
Some types are sometimes used thematically, like Water type with Urshifu, Steel type with Lucario, or Rock type with the fossilmons. But they're mostly literal.
It's when types get thematic that it gets weird. Most Dragon-types are big lizards and most Fighting-types are martial artists, but Dragon can also be used to represent overwhelming primal power (or just a cultural reference), and Fighting can also be used to represent justice.
Psychic, Ghost, Dark, and Fairy are the ones where it gets weird because they aren't as literal.
Many Psychic types have psychic powers, but others use magic, are from space, or are just smart.
Ghost types tend to be dead, but are sometimes incorporeal, mysterious, and in the example of Decidueye, just sneaky.
Dark types are always thematic. It represents evil and dirty tactics, and it never represents a literal element.
And Fairy is all over the place. It's cute things, pink things, nature, the moon, magic, yokai, candy, and sometimes even things based on actual fairy tales.
The reason why there's ambiguity here is because those types are so vague.