The pigments used in modern oil paints are different than the ones used in older paints because they are more lightfast and vibrant.

A. are different than the ones used in older paints because they are

B. differ from those used in older paints in that the modern ones are

C. are different than those used in older paints because the modern ones are

D. are different from the ones used in older paints on account of being

E. differ from the ones used in older paints because they are

I do agree with the OA from Mgmat but not sure whether the explanation is correct..
The MGMAT says that the statement "...X are different than Y because..." is misleading due to the use of "because": it implies that because the pigments used in modern oil paints are more lightfast and vibrant, they are different (in some unspecified way) from the pigments used in older paints. However, the intended meaning is that the pigments differ "in that" one is more lightfast and vibrant.

I think whatever in bold is really what the sentence wants to communicate.. Isn't it?
The sentence following the the bold also describes the same.. Then why is the usage of because is wrong over here...