You also have to include the cost to maintain the fossil fuel plants that back up the fossil fuel plants, in the fossil fuel analysis.
The UK National Grid maintains a "spinning reserve". This has to be big enough to cope with a couple of large fossil fuel or nuclear plants going offline suddenly, which does happen from time to time (and there have been blackouts when there was not enough spinning reserve when two power stations went offline - for unrelated reasons - within minutes of each other). From the point of the UK National Grid, nuclear, coal and gas are seen as "intermittent power sources". Sizewell B, one of the largest generators in the country, could go from full capacity to zero in an instant, without any warning, if a problem occurs - and suddenly you're without a terawatt of generating capacity. Wind power on the other hand doesn't suffer this problem, wind generators are small and numerous and the loss of one of them doesn't have that kind of impact since at most they are only about 2MW each. Over the period of the next hour or two, wind is also extremely predictable. The wind doesn't just unexpectedly stop blowing. Also in the UK, it tends to be windiest when power demand is highest, those dull winter days when it's doing horizontal rain and everyone's got the lights on.
Of course you still need an alternative for when the whole country is under a high pressure system and there's not much wind at all. But any power generation system alone isn't a silver bullet, that's why we don't just have solely nuclear, or solely gas, or solely coal, or solely oil - we have a mix of different fuelled generation.