Smurfette was not a “real smurf” until Papa Smurf cast his own spell (which also made her blond, for some reason).

When Sassette was created by Papa Smurf he used Gargamel’s spell and then his own, just like with Smurfette.

That spell might be one the true genius creations of Gargamel, but he was missing the “second half” of it.