I have noticed this in supermarkets as well. Either it is to trick the consumers but could equally be consumers would rather buy 6 cans of X in a pack than go to the effort of taking 6 cans individually off the shelf.
I spent my first 7 years out of college building business intelligence software for grocery retailers. My guess is that when it happens, it's a f-up rather than something nefarious. Especially with Amazon, whose seller interface is pretty much user-hostile.
64oz Store brand half and half: $3.50 (avg range $3.25-3.68)
32oz Store brand half and half: $1.50 (avg range $1.25-1.68)
Um, wat?