This is all due to the immense hype of Kreia's little quote. It has spawned a disease. Look. I want to bring this up about Kreia's quote, okay?
1. Kreia is a dubious source. The woman's been described as Palpatine's other half. She's a damn good manipulator, tactition, and a liar.
2. Kreia, despite immense wisdom and power in the Force, is not all-knowing. No one is always right in the Star Wars universe. Yoda and Sidious and all the others have been wrong during many occasions.
3. Kreia is an Expanded Universe figure commenting on issues that tie into movies. People have thrown the fact up that "Sidious isn't most powerful because Lucas hasn't said so." Fine then. Lucas hasn't agreed with this quote, thus it must be wrong. Point moot.
4. Kreia is speaking of Ancient Sith from beyond KOTOR times. That means people then may not be as powerful as those of ancient times. That means the "Ancient Sith own all" rule isn't necessarily privy to Revan.
5. There are always exceptions to the rule.