>Erika uses three of these Pokémon in Single Battles, four in Double and Rotation Battles, and all six in Triple Battles. Erika will always lead with her signature Pokémon, Vileplume.
-Bulbapedia
It is indeed mentioned that Erika's signature Pokemon is Vileplume. However, I believe it is actually Gloom. This is because, in Pokemon Yellow as well as G/S/C, she uses a Bellossom. Maybe she mistook a Sun Stone for a Leaf Stone, or maybe GameFreak wanted trainers to encounter more number of Pokemon.
The exact reason, I admit, is evasive, but the fact as to why it is is HG/SS, is simply because that is how it was in the original Gold/ Silver/ Crystal.
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