I agree with Sakhidad Haatef : Farsi and Dari are not the same language. I have had this conversation with many people. When you have a conversation about some thing like this issue, you know you are right about it , but the person that is with you does not agree but he also knows that you are correct about it but he doesn’t agree. That is when it makes you more frustrated although you are trying to be patient.
So I am going to explain it here (just to save more argument) :
Farsi : Comes from the word Fars, which is now Iran
Dari : Comes from the word Dara , which means the long mountains of Hind-Kush
That seems a big difference. We say کچالو They say it سیب زمینی .