Hi there,
and Welcome here,
SO check out the Threads by our Power Ninja on here @msfujino go look at those threads (3) you will be able to do all kinds of power saving tricks he has discovered. The extra 1ma. may be the onboard battery PMIC, you can look at the schematic from the wiki page and see your power paths. It should help with your endeavor.
You are very close, with the right settings you can get single digit >5uA 
Talk about sipping power… LOL the Nrf 52840 does a great low power job. Check also the online battery life calculator to get some inciite on size and duration.
HTH
GL 
PJ 