There is a difference between research figures for boar taint in Denmark and other European countries. Why is that?

This difference arises because of the existence of different parameters. In the Netherlands the average is about 3-4%. This concerns the so-called hedonic parameter for boar taint, i.e. as experts perceive it. The 10% in Denmark stands for the percentage of animals having a level of skatole and / or androstenone which is above pre-specified limit values.
For skatole the limit is >0.25ppm. At higher levels of skatole and androstenone, the chance of boar taint increases. It may be that human experts smell boar taint at 3-4% of the carcasses, while 10% of those carcasses exceed 0.25ppm.
When Danish or German experiments refer to boar taint, it generally concerns the levels of skatole (and sometimes androstenone). This is not the same as the percentage of boar taint that is perceived by humans.