Dari and Farsi are exactly the same language with some small amount of vocabulary differences in iran, afghanistan and central asians, and off course the difference in accent. There is no any grammatical differences between, as they are the same language. we shouldnt forget that 4 decades ago the language in Afghanistan used to be called Farsi. they changed it to Dari for some political reasons which doenst have much of acadamic value. Having different vocabularies in differnt countries which speak the same language is very normal, you can see this in every part of the world, as you mentioned arabic language.