Eggs

I got 4 new chooks (2 in May, 2 in August) because I lost one and the other two older ones weren't laying. Then one of the new ones went broody for WEEKS so it was maybe 2/3 eggs a day all summer with the older Bluebelle laying softies every night which broke through the rungs. What a waste!
Anyway....since breaking the broody and everybody getting settled as a flock the two older ones have started laying properly again. The Bluebelle is producing knobbly hard shell eggs every day and the Magpie is laying occasionally (one egg was 96g!!) The broody Speckledie is also laying again.
This means I'm getting 5 eggs a day pretty regularly (haven't had the magic six from six girls yet) which is fantastic considering it's Autumn.
I genuinely thought the two older ones were past it. Why was the Bluebelle laying softies for so long and now laying normally? Is it a calcium absorption thing? (Kidneys?) They've always had oyster shell available.

No...this was the old Bluebelle who has laid regular as clockwork for 21/2 years then started laying softies for months nearly every day and is now producing hard shells nearly every day!! What a clever girl!
That's why I got the new ones....I thought she and the Magpie were finishing!
The young ones are good as gold....5 eggs again today from 6 hens!