DD results do not produce a systematic bias, for most reasonable definitions of systematic bias. It as just as random as the "noise" from the simulated frequency tables. If my opps bid a ridiculous 7NT that happens to make on 3 finesses and a squeeze, I shrug and move on to the next table. The other pairs in the room are quite likely to have more normal results. However, the DD result is 7NT=, so now instead of, at most, one pair (+their opps) getting a ridiculous score, now everybody in the room will get a ridiculous score. It's not difficult to imagine that all of the pairs that happen to have 7NT on will have a near-zero chance to win the tournament, as they have to make up 34 imps (2 times 17) in perhaps as few as 24 boards.