OK what am I doing wrong here?
I don't know - what makes you think anything is wrong? SOMETHING has to be the major user of power - would you feel better if it was Android system?
I don't see anything wrong with the (limited) information that you provided: the battery is at 75% after 15 hours in this cycle - that seems reasonable.
Next time please also provide screenshots from the History details, Screen on and from the Google Play services - that'll provide a better big picture of what may be going on.
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