Correction: They don't claim 100% compatibility. That's their goal.
"CentOS 2, 3, 4 and 5 are built from publicly available open source SRPMS provided by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policies and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork)."
www.centos.org - centos.org content