I think some of the time for this, is they have X of common standard sized box, and Y of less common smaller/differently shaped box, and eventually they run out of Y sized box, so they go down the list of close matches until they find a box in stock at that particular packing station. I've ordered the same thing (baby formula premix ready to drink, fairly large, fairly heavy) in sets of two for several months and it's been shipped at least four different ways that I can remember; I doubt the bin packing algorithm is changing that drastically from week to week.