[NTG-context] \tfx and \tf having the same size

Hello, I have noticed that \tfx and \tf yield the same size. For example in the sample below: \tfxx A\tfx A\tf A\tfa A\tfb A\tfc A\tfd A However, the documentation states that tfx should be 0.8 size of tf. (http://wiki.contextgarden.net/Font_Switching) Please, what did I get wrong? Thank you. Best regards, Tomas