Tims on holiday for a week, but i will try to help you.
I doubt its calibration ,but you could prove this by doing a simple LS test as same signal and check this gets a good result.
(i asssume the unit has latest firmware and passes self test?)
Also, please note that the OC and SC compensation should be done WITH the DC1000 connected to the fixture, and OUTPUT ENABLED (but with DC=0.00)
As its such a high frequency, check Earths on the AT and DC1000 too are good.
Hope this helps; let me know how you get on.