Yes, it's coming from centos-userland-release :
rpm -q --scripts centos-userland-release :
postinstall scriptlet (using /bin/sh):
# Detecting if rpi or generic kernel
egrep -q 'BCM2709|BCM2710' /proc/cpuinfo && echo rpi2
>/etc/yum/vars/kvariant || echo generic >/etc/yum/vars/kvariant

So normally it's detected correctly and so put the correct rpi2 value
for kvariant when installed on a rpi2/rpi3