I've had that happen before, its one of natures wonders I think
they are sterile but the hens will lay eggs.
The chicken x Guinea fowl has a very poor fertility rate and therefore a very low hatch rate.
The chicks survival rate is better if they take more of the GF characteristics rather than the chickens and the eggs will hatch out either at 21 days or 25 days.
GF will also cross breed with Peafowl occasionally