What happened is fairly simple. In Warcraft III, we're told that the eredar were a race of sorcerers and warlocks whose corrupt magics date back to the dawn of time, devourers and corrupters who ran afoul of Sargeras before the titan went mad. In fact, in the original story, it was his encounter with the eredar that started Sargeras down the road that would lead him to go mad. At this time, no mention was made of the eredar being in any way related to the draenei, nor were the draenei depicted as anything beyond the deformed model used by Akama. Even when World of Warcraft debuted, the few draenei in game were known as lost ones, such as Magtoor or Kum'isha the Collector, and they were nothing like the draenei we have today.
The obvious lore contradiction with Sargeras and his encounter with the eredar was clearly documented in the Warcraft III manual. I wrote those bits about four years ago, and to be totally honest, I simply forgot.
-- Chris Metzen, Metzen on Lore
And then the mistake happened. Mr. Metzen, in preparation for The Burning Crusade, came up with a way to link the draenei of Outland to the eredar and in so doing, completely contradicted what he himself had written in the Warcraft III manual. And in so doing, he made the game as a whole much stronger.