On a fully patched C7 machine...
% cat /etc/redhat-release CentOS Linux release 7.1.1503 (Core)
% cat /etc/os-release NAME="CentOS Linux" VERSION="7 (Core)" ID="centos" ID_LIKE="rhel fedora" VERSION_ID="7" PRETTY_NAME="CentOS Linux 7 (Core)" ANSI_COLOR="0;31" CPE_NAME="cpe:/o:centos:centos:7" HOME_URL="https://www.centos.org/" BUG_REPORT_URL="https://bugs.centos.org/"
CENTOS_MANTISBT_PROJECT="CentOS-7" CENTOS_MANTISBT_PROJECT_VERSION="7" REDHAT_SUPPORT_PRODUCT="centos" REDHAT_SUPPORT_PRODUCT_VERSION="7"
In particular note the version ID is 7
On a RedHat machine: % cat /etc/redhat-release Red Hat Enterprise Linux Server release 7.1 (Maipo)
% cat /etc/os-release NAME="Red Hat Enterprise Linux Server" VERSION="7.1 (Maipo)" ID="rhel" ID_LIKE="fedora" VERSION_ID="7.1" PRETTY_NAME="Red Hat Enterprise Linux Server 7.1 (Maipo)" ANSI_COLOR="0;31" CPE_NAME="cpe:/o:redhat:enterprise_linux:7.1:GA:server" HOME_URL="https://www.redhat.com/" BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Red Hat Enterprise Linux 7" REDHAT_BUGZILLA_PRODUCT_VERSION=7.1 REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux" REDHAT_SUPPORT_PRODUCT_VERSION="7.1"
Here the version ID is 7.1; different to CentOS.
Is this a bug or is it deliberate?