it seems like coke and pepsi are never on sale at the same time. Is there an unwritten rule between them?

Answer 1

Actually yes -

The Major Grocers now have the Biggest Company help dictate how to run the shelves in each category.

In the Soda Aisles it will either Be Coke or Pepsi - depending on whos produt moves there. That comapny will also make the "Off Brand" sodas for that store.

Then, they will alternate End Caps (the end of the aisle displays) and sales - with the exception of a few weeks when both can be on sale.

The Market in Grocery has not been an open market in years.

I once represented a company placing a new product in Grocery. You have to basically, Bribe the Buyers, Pay in advance for Warehouse and shelf space, and afew other costly things before you can even get it on the shelf.