It's not Ismene (they have an extra set of petals/sepals whatever that curl back heavily).
Most propably it's two different species, a variation in the genetic make up within the species, or - what I think it really is - age of the flower. Older flowers spread open much like the first photo and younger ones look more cup shaped like the second.
I've also noticed on some of my H. harrisiana having differences in flowers. Some had short pistils and others had much longer ones.
It all adds to the natural diversity.
