Dresspheres are key items in FFX-2, and the Songstress dressphere is probably the most special of them all: it is the only dressphere that connects to the main plot.
Official sources stated that the songstress dressphere was created from Lenne's memories, but I prefer the word soul if you will. As proof, a living person's memories are required to construct a solid image of the deceased, but Lenne was able to separate herself from Yuna's dressphere during Shuyin's last moments on her own free will. How? Mere memories can't move on their own like that, let along being carried over from a thousand years past. On top of all, Lenne is not an unsent like Auron nor a "shadow" like Shuyin. The only explanation left would be that Lenne's feelings were strong enough to attract the pyreflies momentarily, and having Yuna's power synchronized with her own, she was able to appear in human form for one last time.