Maybe it is how gamefreak wanted them to be like.
But Smeargle is best known for its ability to use the move Sketch, which permanently copies the last move used by the opponent. Due to this, it can legitimately have any move in the Pokémon games except for Chatter, and it can pass on any Egg move to Pokémon in the Field Egg group.So it can learn any 4 moves after it sketches it.So smeargle does not have 1 but 4 moves.
Ditto uses the move Transform to copy the opponent's moves, types, form, and non-HP stats. It can breed with any Pokémon other than the Pokémon in the Undiscovered group and itself, including male Pokémon and some genderless Pokémon, to produce Eggs of the other's species.This makes it very useful in game.If ditto uses transform on the Pokemon it sees,it gains its moves as well where ditto will have 4 moves that time.So not 1 after transforming.