The loss of 1/3 to 1/2 of the bees that went into cluster during the winter is not that unusual. The important thing is to emerge with a queen and a cluster large enough to respond to spring stimulus. This can be done with a softball size cluster come spring. I've successfully overwintered bees in a double stacked medium nuc (2 medium 5 frames) and then had it go on to grow to 4 medium box brood nest plus surplus over the course of a season. Late swarms or splits are ideal for this approach. It is also a great way to develop survivor stock.
Currently I have bottemless hives facing a south wind and experience temps averaging the mid 20's for a week or more at a time and haven't lost hives except for blow overs. Hives that get blown or knocked over during cold periods lose the ability to retain heat in the cluster as most of the boxes become exposed to the air at both top and bottom.
A cluster of bees will survive if they have sufficient stores, the hive integrity doesn't get compromised (knocked over), they don't get wet, and the temps don't dive to the basement for 10-14 days straight so they can't retrieve stores.