You also have to factor risk of spoilage. 2 units of milk is cheaper than 1 unit. But what is the risk of .5 units being thrown out since you normally don't use 2 units within the time it takes to spoil. You could increase consumption to not throw it out, but then you need to balance that out or all you did is increase the amount you spend on groceries and the calories you consume while attempting to actually reduce the money spent on groceries.

Stores are good at using sales to trick us into comparing prices as if we were going to buy on sale items anyways (in which case money is definitely being saved) instead of comparing it to what our actual behavior would have been had the items not been on sale.