Free clones of free clones
Scientific Linux is not a clone of Centos. It's also a derivative of Red Hat's source (not quite a clone, for significant reasons).
CERN(*) depended on the old Red Hat free-to-copy model. When Red Hat started charging after RHL 9, CERN had a problem. (Methinks someone at Red Hat didn't understand that CERN had thousands, perhaps millions, of systems embedded in apparatus, and really could not countenance any per-CPU charging scheme. I suspect that if Red Hat had offered CERN a no-support unlimited-copies license at a reasonable price, i.e. the status quo, they'd have paid for it, and the rest of us would be poorer for it).
Anyway, they didn't, and CERN took the only route that they could. Changing horses was not an option. Taking the source, and building their own distribution, was an option. CERN has a lot of very smart IT guys. So Scientific Linux was born (with the most inappropriate name of any Linux distribution).
Maybe it was a clone on day one, but they take the attitude that if something is needed for CERN that's not in Red Hat, it goes in, and if a bug is troubling CERN, then they fix it (even if Red Hat hasn't, or won't). However, they prefer to avoid divergence. From an ordinary use's point of view, you'll find it hard to tell the difference between Scientific Linux and Red Hat. The most obvious change, is that a default Scientific Linux install has automatic yum updating turned on. The next most noticeable thing is that SL has a fair number of (science-related, optional) packages in the distribution repositories, which are not in Centos or RHEL. I'm told that the SL kernel has a few extra things built in or removed into modules, but I've never run into anything that works on Centos or RHEL that doesn't work on the corresponding SL.
Centos used to to claim bug-for-bug compatibility with Red Hat, but since RHEL6 that has become harder for them (different build tools). Anyway do you really want to suffer a fixable bug just because some other distribution hasn't yet fixed it? So now Centos is also not quite a clone.
Perhaps it's like evolution. They're strains or races, not yet different species. The environmental change that would cause a speciation event (or a fork) has not yet happened, and hopefully won't.
(*) CERN implies "and Fermilab", everywhere.