Hi there, welcome to BYC!
There is no cause for alarm, the brown color is genetic and not health or calcium related. It is very common for them to start our lighter and get darker. And the shade can vary throughout the season... more laying equals lighter SHADE. Slower can equal darker. In the Marans breed they consider it an average color around the 20th egg that's laid. All eggs are white underneath (except true blue goes all the way through). The shading is one of the last layers to go on and you can even rub it off if you catch it while still wet after being laid.
White layers also have white ear lobes. Cream layers often have red with a bit of white on them. Brown layers have red lobes.
If you got this California white from a feed store, my guess would be a mix up in labeling... and possibly it's a white rock?? But the black spots in her feathering lean more towards your current understanding. As does your laying age.
The other option has been noted and there was some cross breeding (which is what production birds are) and the brown egg gene got bred in somewhere (to the California grey sire)... brown is dominant to white in eggs, so is blue. So probably chalk it up to "hatchery" quality.
White eggs are cool. Those are lovely too, congrats on your first eggs!