HT Why can not the frequency of 900?

I have a 3800 + X2 2.0G of the U, this time in the super-2.41G, down to 533 of the 667 memory, K8HT is 3, PCI-E is 100, not pressure, according to the relations between the HT bus accident rate as = X5 And then drop a little X4, it should be 2.41X4 = 964MHZ ah, but I can not on the 900, 800, is also not on, now at 700, very stable.Until it is AUTO, to a maximum of 500.
Please cattle I help people solve the confusion.