According to WhatBird, house finches are year round in southern B.C. and Saskatchewan, so it doesn't seem too strange that you'd see them at this time of year in Alberta, depending on the weather patterns.
What I really wanted to say, though, is that I've noticed the exact same phenomenon with my local house finches, and I'm in Missouri - smack dab in the middle of year-round territory. All summer, the house finches have been hogging space at my feeder, and when September hit, they just disappeared. Gone. Finally, this morning, I saw one for the first time in a couple of weeks. We haven't had any rain lately, and the creek in my backyard dried up, so I blamed their dissappearance on that.
Maybe your finches are your regular summer residents who haven't flown south yet but have been responding to some other condition - weather, noise, other animals ...
Amy and Merton of Cat