In their final song, "The Day Before You Came", Agnetha takes the lead, with Frida's contribution being an operatic line running through the instrumental sections (there's one in the middle and one at the end). Since Agnetha is a soprano, and Frida a mezzo-soprano, would it not have made more sense to have Agnetha do the operatic line instead, since it would have been easier on her vocal cords than it would have been for Frida?
Frida is more skilled at opera singing(a very different style than pop) as this clip shows. A lot of people with good voices and high ranges can't sing opera because it takes specific vocal training.
Moreover, what Björn & Benny composed for Frida to sing fits a mezzo-soprano range better than a soprano one. Having a higher voice doesn't necessarily mean that's the right voice for the part.