I read that as well, but know there can exceptions to this rule. This spring I had a hen lay an egg in the late afternoon on day 1, and then lay a second egg first thing in the morning of day 2 -- so somewhere around 12 to 14 hours apart. She did this on two different occasions. She is the only blue layer in that coop, so I know the eggs were hers. The first time it happened, I observed her laying the afternoon egg, collected it, and then found a second egg in the box when I opened up the coop in the morning. The eggs had normal shells. I can't explain it, but I know it happened.
I've also had a new layer last fall, struggle quite a bit when she came into lay during a heat wave. She dropped out 1 thin-shelled and 2 soft shelled eggs during the course of one afternoon.
I don't think these are normal egg laying behaviors, and probably indicate something a bit amiss with the pullets. Both of the hens I mentioned above are still laying -- the blue layer settled into the normal 6 days on, 1 off pattern, and the other hen is pretty regular, although she still tends to struggle in hot weather with thin shelled and shel less eggs. Both hens also lay huge eggs, occasional double-yolked eggs, and have that ratty look of over-achieving hens for whom laying is taking a lot out of them.
In your case, I would be inclined to think that is stress, rather than a calcium deficiency. Putting some electrolytes or nutradrench in their water for a couple of days wouldn't hurt. Other than that, I would watch and wait, and hope her system stabilizes.
