The limits are often covered pretty misleadingly. If you were to just order a truckload of American butter and import it, you'd be subject to the over-quota tariff regardless of whether the cap was reached. In order to get the preferential rate, you have to file a formal application to use the quota (https://www.international.gc.ca/trade-commerce/controls-cont...), which is subject to all kind of restrictions and limitations. In particular, grocery stores aren't allowed to use the butter quota at all, so if you have some fancy butter you'd like to convince Loblaws to stock you'll always have to pay the higher rate.

(I would emphasize that I don't think this is bad in some cosmic sense, and Canadians have every right to keep the shelves stocked with Canadian butter only if they would like.)