Guess it all comes down to the luck ( or misfortune ) of the draw :
Economic ties between Canada and the U.S. are close, the number of parcels and packages that get shipped to and fro commercially or privately must staggering. Add the fact that many citizens of both countries have family roots all over globe. Add the fact that anyone can order stuff with a few mouseclicks from a store 3 miles or 3 continents away. Means there is a tidal wave of packages and parcels flooding in and out every day.
Customs got bigger fish to fry than a few bottles of liquids for private recipients. Of course, if your package happens to end up in a random look-see, it might get returned or seized, but judging from all the posts, the stats are clearly in your favour...