It’s not, It’s off by a factor of at least 3 and more likely a couple of factors of that. That system isn’t pulling 2 watts.
Adding a UPS into the mix would only increase the power draw. Going back to the external power supply would give better results.
It definitely isn't off by a factor of 3, I've tested the Zalman with known loads it is reasonable indicative even at low wattages.
The UPS isn't to improve efficiency just saying it is hooked up via that now - I'm using a DIY modified Eaton 3S Mini - I put quite a bit of time and testing into this setup including modifying the UPS with an ATTiny85 controller and writing custom software for it - part of which has been measuring the power draw under various different circumstances to make sure the UPS was suitable:
EDIT: PS we are talking about full idle wattage here where drives are spun down and the OS and hardware is in its lowest non-standby power state, not "active" idle which it is in most of the time, other than when unused for long periods, which is somewhere around 4-7 watt depending on factors like what Windows is doing.
EDIT2: Also if we were talking the N5105 which replaced it you'd be right
even with a high efficiency power adapter that will pull around 7 watt from the wall with a lot of tweaking and nominally 10-12 watt idle
which is an annoying step backwards.