An interesting observation. I have just been revising clothing and to my surprise I observed that in lesson one all the pictures that illustrate "Jakke" are what I would call a coat not a jacket. Actually not all! One of them is what I would call a cardigan. It seems Norwegians have a more fluid interpretation of the difference between Jacket and Coat.
Hva er forskjellen mellom jakke og frakk?
According to other comments I have read on the forums, the most basic difference is the length. En jakke is a jacket, which tends to be a shorter coat, like a windbreaker, around waist-length or so. En frakk is a longer coat; I believe one of the comments said it should come down to at least mid-thigh length, or longer. It generally corresponds to what we'd call an overcoat in English, and is worn to cover a dress or fancy, dressy clothing. There may be some difference in materials as well, but I think the length was the main differentiating factor.