Here https://quay.io/repository/coreos/hyperkube?tab=tags you can see that there are two tags for 1.6.6:
What is the meaning of 0 and 1?
What is the difference between the images?
According to https://github.com/coreos/kubernetes/releases , these are identical images, but one was re-tagged to allow new hyperkube image build at quay.io . For details, please have a look into provided link.