The only thing I can think (and this is just supposition) is that by not having all the people from the same section of the plane show up at the same time, the packing efficiency in the bins is better. Maybe having the bags put into a particular bin over time, rather than all-at-once, results in people looking and putting things in "better" so that the space is better-used. Maybe.
Cheers.
Maybe. I suspect the author misunderstood what he was told by AA, or whatever was originally written was botched during editing.