Some (but not all) thin shelled eggs

[BellaM]

I have a small flock of dual purpose indigenous chickens. They get free choice layer feed, kitchen scraps and they free range during the day (cooped at night). I also feed their eggshells back to them (cleaned, dried and crushed). The hens lay white, pinkish and brown eggs. The pinkish and brown eggs have good strong shells but the white ones are very thin. They don't break in the nest or anything like that but are noticeably thinner.
Any thoughts on what could be causing them to be different to the other eggs shells?

 

[CluckerFamily]

I would think that if they are eating the eggshells everything would be okay.
Maybe stop with the kitchen scraps and any treats to see if there is a change.
You could also buy a small bag of oyster shells to see if that will make a difference, I'm not sure if them eating the thinner shells has anything to do with it. I'm stumped!