unless the pet store got her directly from a breeder or were supplied with her birth date, they most likely had no idea of her actual age. i wouldn't put faith in pet store employees knowing too much about the animals they sell.
what is her weight? that might give us a little bit of a clue as to her age, but it won't be super accurate. from my reading, chins are weaned at about 8 weeks, so if you got her at the supposed '2 months old' then she'd still be pretty small.