Yes I was thinking the same thing. Certain factories may have worse quality control, and those sets end up in different regions from sets that come from better production facilities
One thing they do on the big sets is give you extras of the small stuff. Like if you need 10 of a 1x1 tile of a certain color, maybe they give you 11, or even 12.
The biggest limitation to that is that the selection of extra pieces is probably not as random as the set of missing pieces, especially since they focus on smaller pieces - presumably to accommodate the ones you lose after buying a full set.
In other words, I would not expect the distribution of Lego sets that are missing pieces to be evenly or randomly distributed.