For example in Germany you are considered "inactive" (neither employed nor unemployed) in that age group if you:

• go to school
  
• go to university
  
• do compulsary service like the draft

And students in Germany can work for 19 hours a week (or full time during spring and summer breaks) without losing their student status. Meaning that you only have to pay the wage tax but no payments for unemployment insurance and social security. And you still stay in the student health insurance.

If other countries count such a student as "active" and employed, that would change the numbers.