Rainbow Foods and the 'new math'

On Sunday I went to the Rainbow store at 1276 Town Centre Drive in Eagan, Minn. (Store #8815.) On sale this week are Healthy Choice dinners @ five for $10 (limit five). I bought six of them because I had a coupon from Healthy Choice for one free frozen item (maximum value of $5.29). When I got out to the car, I checked over the receipt and found that I was charged $10 for five dinners and the regular price of $3.69 for the sixth dinner and then was given a credit of $2 (the sale price of one dinner), meaning that I was charged $1.69 for the "free"item". I went back to the service counter, and was informed that when several items are purchased, the coupon for the "free" item is for the lowest priced item. That may make sense to someone but not to me because, as I stated earlier, I ended up paying $1.69 for a dinner that should have been free.