The Utilization Summary works differnt than I expected, and I can't figure out why.

I think this image summarizes my question:

In short:

• I have 5 working days (40 hours) per employee as ''Available hours''
• And my tasks are split into working days, where ''Hours required'' is calculated as [WorkingDays]*8
• Now in the example above I have a ''task'' of 15 working days, spread over 3 weeks, resulting in 5 days per week. 
• I would expect to get three weeks with 100% utilization, but instead, I get three weeks with 300% utilization. It seems like Utilization Summary takes all Working Days for every week the task occurs in.

How can I solve this?