>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.
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.