The wattage usage display on my UPS; the 2750 is on the AsRock C2750D4I and the the 2758 is on the SuperMicro A1SRi-2758F. I've never see either dip above 24w, nor has either ever consumed less than 24w, granted the 2758F is running Sophos UTM, which isn't resource intensive.]
The Serve the Home testing found the C2750 and C2758 platforms (i.e. the whole system, including motherboard, RAM, network adapters, attached SSD, etc) using roughly 17W at idle and 33W when under heavy load. Your values are in between those, which suggests your usage isn't fully loading up the CPU to force it to use maximum power. It is impossible to know how much of your power is consumed by the CPU, and how much by all the other stuff, such as chassis fans, hard drives, SSDs, etc.
Note that their testing (if I am interpreting it correctly) found that the RAM was a very significant percentage of the total power consumed (28W of the 33W total platform power under load was due to RAM - this seems crazy high, so perhaps I am misinterpreting their results).
If your displayed power consumption never changes, I bet it is close to idle power. What is the C2750 running, and are there any hard drives or chassis fans on that system?