Perhaps it was discovered by an individual on OS X, reported (by the individual) to Apple, reported (by either the individual or Apple) to CERT, then CERT looked at it, found the other affected operating systems, and reported it to them?

There's a number of ways this particular "ordering" could have occurred.

It's possible it was reported to Apple because they have a vulnerability bounty program, so they individual who reported it could get a reward. That might create an incentive to report it to larger targets who may run larger bounty programs.