In a test with 6 sunstrips from the MA3 library I have the following result with the command 'at 100 fade 4 delay 0 thru 16':
Group 1, fixture 1 thru 6: fixture 1 at full then fixture 2,3 thru 6 with delay as expected. 6 fixtures in a column in selection grid.
Group 2: same selection but with command Down. A 6x10 matrix in selection grid.
Same behaviour as group 1.
Group 3: Same selection but with down; nexty. First instance in each fixture with delay, as expected. A 6x10 matrix in selection grid with 1.st instance highlighted.
Group 4: the same fixtures selected with lasso from left to right in 3D: each instances in fixture 1 with delay , then each instance in fixture 2 with delay and so on. All units in 1 column in selection grid.
As usual I am looking forward to a logical explanation.