As we can see, CentOS fixed their repos so that http://mirror.centos.org/centos/7 returns the latest version we need for vmlinuz, etc..
For a long time they only offered these files under specific version dirs, but now they have deprecated this and we should just refer to $major, not $major.$minor
See the notice at http://mirror.centos.org/centos/7.3.1611/readme