Egg shell with white residue?

My 3 girls are laying 3 brown eggs a day, some are speckled, some not, one of the hens lays HUGE monsters. But one has a tendency to lay eggs with a white residue overtop. It's hard, and part of the shell IYSWIM, and the eggs are fine, I was just wondering if I should be concerned? They are all hybrids, all 6 mo old. And in fine fettle otherwise!

If You See What I Mean..

Thanks, that's helpful! It is from the youngest one too, so that explains it.