Maybe you need to re-read my example. Yes, you are saving the same amount of money...but you are saving a larger percentage of your spend.
You must buy 2 gallons of milk per week...you can't control the cost, but have to spend the money to buy it. You have a coupon for $X off of the purchase. Would you rather spend it when the price is high, or the price is low?
If you are on a limited budget, you might choose when the price is high to help offset the "temporary price increase"
If your budget is not influence by the cost of milk, I would argue it makes sense to use the coupon when prices are low, as you save a larger % of your required expenditure.