Those % that you see are not very correct because when the hero levels up he always gets to choose from 2 skills. One that he has and one new. The % are valid for new skills only and change with every picked new skill.

for example if you have normal hero with 12 skills and 1 is already learned by the hero and he levels up then:
1. There is 100% chance he will get the skill he already has.
2. All other skill % are:
- 12% for the second and third
- 10% for the forth and fifth
- 8% for sixth, seventh, eighth and ninth
- 4% for tenth, eleventh and twelfth skill

or Total 100%


For heroes like Khans and Veterans that have 13 skills.
All skills that are the same as Chieftain are the same % even though they are shifted. This means because the second is Shout or Enlightenment:
- 12% for the third and fourth
- 10% for fifth and sixth
- 8% for seventh, eighth, ninth and tenth
- 4% for eleventh, twelfth and thirteenth

As far as the picked skills selection goes, it is separated 50% vs 50% between Blood rage and Shouting (or Enlightenment) as they both have 12%.