Ah, there's the problem.
Those hens are Red Sexlinks, not Rhode Island Reds. Your chicks are what I would expect from crossing those hens to a Black Australorp.
The Australorp contributes genes that would make the chicks be black all over.
The hens probably have a Rhode Island Red father, and a white mother, so they have both the Dominant White gene (from their mother, turns black into white) and the gene that allows black (from their Rhode Island Red father.) That's why some chicks are able to be all black, while others are mostly turned white. It just comes down to which gene they got from their mother.
Dominant White is a leaky gene, so it lets bits of black show in some places, which is why the "white" ones are not completely white all over.
Edit: I looked more closely at the photo, and there might be some Rhode Island Red hens in addition to the Red Sexlinks. Rhode Island Reds are red with black tails, while Red Sexlinks are usually red with white tails and a sprinkling of other white feathers. The Sexlinks are still the reason for the white chicks.
