From Glitch City Laboratories
A relatively minor glitch exists within the Generation I Pokémon games. Usually the Pokédex should register Pokémon that the player has encountered as "SEEN" by managing the encounter flag of that Pokémon.
However, when the player encounters a ghost in the Pokémon Tower if the player does not have the Silph Scope, the Pokémon through normal gameplay cannot be identified. Regardless of this, the Pokédex still registers the Pokémon which is the ghost as seen.