Yuna looks like Lenne. The dressphere is a recording of a person that Shinra uses with his garment grid to invoke the abilities of the person recorded, kinda like the crystal shards in FFV. Since one invokes the recording using the garment grid, it makes sense that at least part of the persons personality is carried through as well. Example, the berserker dressphere.

But it is obvious that they are not the same person. Look at their eyes for proof.