Copy Data Activity Cost Consumption - number of DIU not taken into account?

"You will be charged # of used DIUs * copy duration * $$/DIU-hour". However, looking at the consumption from recent runs it's seems like the number of used DIUs is not taken into account?

In this example the duration is 2 minutes and 18 seconds with 4 used DIU. The returned consumption is 0.05, not 0.2 as I would expect (3 minutes * 4 DIU / 60 minutes). What is right? 

I've experienced this in both adf and synapse.