Hi all, What is UPS input load and how can that affect the power draw of devices plugged into it?

Context: I have an Eaton UPS. Into it I plugged TP Link smart plug to measure how much my homelab draws (1 truenas server, 1 rpi and a switch). These draw about ~29 W when under low to medium load. Almost every day (different time) for couple of hours, however, the plug measure about 6-7 Watts more (~36 W). I have checked both linux devices and they were doing basically nothing. Then I looked into TrueNAS monitoring and noticed that the start and end of each event is exactly the same time when UPS input load is increased from 0 % to ~6 %.

What is this UPS input load and how is it possible it affects measurements by a device that is plugged into it (the UPS) - NOT the other way around? Thank you