I think your bullet points have pretty-much answered your own question though it's rare to see a hive that isn't good enough to get a colony through the winter. You certainly get the odd duff queen, but certainly not 10% if they come from reasonable stock and have headed a colony going into the autumn. I suspect the main reasons are lack of attention to the last two.
I've lost one colony through isolation starvation this winter. A small colony that I should have united in the autumn. Mea culpa. Big colonies certainly starve, but that's almost always just insufficient stores going into the winter.
Low Varroa, and more importantly low virus levels, are critical. I think it's really important to get the Varroa treatment (which often stops the queen from laying) finished and still have warm enough weather for another round or so of brood rearing. I prefer to take the summer crop off early, slap the Apiguard on, add fondant - which doesn't stuff the brood box out too fast - and let them use the balsam and ivy for themselves.
Actually, I might go so far as to say that winter losses in excess of the 10-15% you suggest are largely due to poor bee husbandry (perhaps other than the odd marauding black bear that Michael Palmer has to cope with).
One local "beekeeper" asked me for a 12kg block of fondant in mid/late November last year and - at the same time - asked whether it was too late to treat with Apiguard I don't known if his colony has made it through the winter.
Bookmarks