Yes, all cats are different. No doubt, some will not be interested in attacking and chasing hamsters. However, given that your cat did kill your previous hamster Botan, I would say that she unfortunately is not one of them (again, I am very sorry for your loss. I do not mention this to make you feel guilty in any way, but just to give a reason for my stance here).
Ham ham Botan is dead
Due to this, I would personally keep the cat locked out of the room in which you keep your current hamsters. However, you say that you cannot do that and that is your choice. I hope no further mishaps befall you.
As for the question at hand: hamsters which have been raised in captivity do not know that cats are predators, so may well not display any fear. I do not think that this is especially unusual.