[CT-3] Measured Wh not correct?

DT2022

I am testing with the CT-3A some powerbanks.
With a load of 10W everything seems to be fine. At least the values are as expected.

But when I lower the load to 2.5 W (5V@500mA) then someting seems to be wrong.
I tested at the moment only one powerbank (37Wh) with a lower load.
I discharged with 10W and measured 32.5 Wh (87% efficiency).
Some days later I discharged with a load of 2.5 W and measured 38.1 Wh

Is there anything wrong?