A great example of buyer behaviour in action
Hvr.
If you sit people in a room, and give them the two options on paper, most people would buy the 30 can pack. The product (in the can) is exactly the same. It's a consumable good with a long shelf life. A rudimentary analysis suggests that you should buy at the best price per unit, and maximise the number of units purchased at this price (within reason - your cupboard is only so big, and you may have cash flow concerns).
BUT - a huge proportion of FMCG purchases are not well analysed rational decisions.
Most consumers don't analyse small buying decisions. They are busy / distracted / whatever. A few things to consider for this example - 24 can packs have more shelf space (generally much bigger on the planogram), 24 can packs are easier to carry, "I've always brought the 24 can pack", "oh Coke/Pepsi is on special, I better get some" (but not paying attention to the specific line).
Each of those 4 scenarios can be explored at length as to why the 24 can pack gets purchased.
Part 2 - The check out person points out to the customer that they can get 6 extra cans for $3.80 less. No-brainier right? Why do people still buy the 24 can pack? Of the 4 scenarios above only one holds true when the facts are presented to the consumer. Why then do people keep buying the 24 can pack - shopper fatigue. By the time most supermarket shoppers go to the checkout, they are ready to go home. They may not hear or understand the price difference, they may not care (fatigue - "get me outta here"), they may mistake the price difference for some kind of onsell (and reject it out of hand), they may be embarrassed and decide to take the 24 cans to save face, they may be nervous about "holding up the line" to go and swap 24 for 30, it may be "too much trouble" to swap. In any case, any number of psychological responses may be triggered.
The end result is the higher priced product is purchased.
For the two big chains, specials are set by head office using any number of clairvoyant (and possibly well thought out) techniques.
I think there is definitely some dart boarding (and sales rotations). Suppliers and Supermarkets have specials and cross subsidies all the time. It's just part of the FMCG cycle, and the fabric of the industry.
When a new product is created and released, the supplier will often usually push for advertising, planogram space, possibly an in-store booth for demos/freebies, etc as part of their initial campaign. This can sometimes include or not include discounting. Loss leading, or followup discounting is often a great way to acquire new customers before a price equilibrium is reached.
Why was the 30 SKU much cheaper than the 24 one? Was the expiry date on the larger SKU earlier (tomorrow?) compared to the smaller one?
Unlikely - most supermarket specials have nothing to do with aged inventory. And where they do, it's generally warehouse inventory (or supply chain inventory) that they are trying to clear (e.g. there are too many pallets of 30 can packs, they have outgrown their bin location at our warehouse/DC). Local stores can "clear" individual items if the screw up their stock rotation and things are close to shelf life.
The Coke/Pepsi example is a great one - Let's put a non-core line on special. Sales of the core line go up, even though the price is the same. WINNER. The smart consumer gets a good deal, and the supermarket increases sales of several lines of soft drinks.
Any item which comes in singles, or packs of varying size can often have weird specials. (e.g. individual rolls are on special - It can often be cheaper to buy 4/6 individual roles, instead of a 4/6 pack). But if you want 4/6 rolls, how many people fool around with tongs to get 4/6 individuals?
Sports drinks, energy drinks, soft drinks, etc all fall into this category.
Anyhow, enough rambling - If you want to know more, go do a Masters in Marketing and get a ~"hundred grand" job at Coles or Woolies.