I think you're right on the dot with twelve days which is when you started the incubation. Those are definitely way younger than 19 days.
One thing to note is that ducks can delay hatching until they have enough to sit on (this may or may not be true. I have read stuff saying this happens and other sites saying they don't typically do that.) So my guess is that's what happened-- she wasn't done yet, or she had just finished and was about to start the brood.