Fabric Capacity Metrics - CU vs Duration

We're trying out PBI Premium P1 and setup Fabric Capacity Metrics App for tracking performance.

I have observed several reports having High CU and Low Duration and vice versa Low CU and High Duration. (See image below)

These cases are mostly related to Refresh/Back-end Activities and low dataset size.

I understand how Duration and CU are calculated individually, but can someone please explain what would contriubute to High Duration with Low CU and vice versa?

Is it about individual report efficiency of datamodel + powerquery has to work OR how many jobs are competing for same time slot during refresh?

Thank you very much!