Difference Between Weak and De-Faco Atheism [Spirituality & Religion]

I was looking up on the Dawkins Scale, and I saw the two mostly atheistic claims of agnosticism, weak atheism and de-facto atheism, and I'm a bit confused on the definition that Mr. Dawkins gave on the category. I asked answers.com and they said that weak atheists feel that the existence of God is improbable. Isn't that the creed of de-facto atheists, though? Thanks a bunch in advance!