The Target analytics algorithm tries to catch changes in buying behavior. They are trying to catch customers as habits change, before a new habit cements in the buyers head---with the hope that Target will be the go-to-stop once the habit cements. So buying more at Target wont necessarily garner a coupon. Buying sporadically---a certain theme once or twice a year (birthday + christmas)---or buying during a transition---for example, buying Duplo for a while and then a City set a year or two later---would be more likely to trigger the algorithm.
That said, I probably shop at Target too often. I rarely get these coupons.