DDR(2-4) Training and Length Matching
I start to believe the training can compensate the length mismatch, at least to some an extent. Though just a guess. Laying out a board is costly, so most people will just match the lengths, though just my guess again. I do see a reference board with largely mismatched lengths. Heard of a processor that can compensate lengths by setting internal registers. Though not sure whether those can add to support of my guessed belief.