I'm guessing it depends on the strain of garlic and growth cycle.... They may just be harvested sooner. I usually plant late September into October too but it was straight up HOT into the last week of November.. I ended up waiting, as you did, and planted in early December. A couple of strains, of garlic and some shallots didn't last that long in storage... I ended up buying more "seed" stock of the garlic I lost. As for the shallots, I've given up on those varieties because they didn't produce well and bolted from last year's warm winter... This winter seems to be very cold and wet and my garlics and other alliums seem to be loving it...

There seems to be evidence that bulbils are best not planted too early, but cloves are another issue. I normally plant in early to mid October here in Minneapolis. Some years the ground is already frozen by then although not normally. This year the ground did not freeze until into December and I got a whole lot of top growth. I am not going to worry about it. Won't do any good anyway at this point. I do not expect problems, at least not from all my varieties, some of which have certainly weathered this kind of weather before. Varieties that cannot take the variation in my climate will get culled. Simple as that.