The results are presented as though all turbine models were equally distributed across all Northern European wind regimes. Of course they are not. Accordingly the results can only be used to gauge general trends in specific yields and should not be used to compare one wind turbine model against another to any degree of precision. Interestingly, as more performance data becomes available, variable speed advocates are shifting their emphasis. They now down play the performance advantage of variable speed, instead stressing variable speed's presumed ability to reduce loads on the drive train.
Engineers widely agree that variable speed can reduce shock loads to the drive train in gusty winds.
This benefit translates into increased longevity or the ability to use lighter drive train components than otherwise possible. Both benefits produce obscure results. There is no way to measure the veracity of these claims with publicly available data, certainly not today. Twenty years from now we can count the number and cost of gear box replacements between variable speed and constant speed machines. In the meantime, though, we only have differences in specific yields to verify product claims.