That is normal. The interface is only specified up to 128 Kbps, but it will normally do 256 Kbps quite happily provided it is externally clocked. But if you set it up as a DCE, then I think the config will only allow you a clockrate up to 128 Kbps.
But I'm not sure how it behaves if you have both channels going at 256 Kbps at the same time, in both directions.
Kevin Dorrell
Luxembourg