I have been comparing against my utility bill for the past couple years at an hourly level. Even after having Sense measuring both usage and solar and doing the net calculation, I am seldom off by more than a % or two. When I am off, it is because of dropped data.
I would suggest that you try analyzing hourly or daily if your utility offers you the data. You might find that:
you are experiencing some data drops
you have a parallel sub panel that you are not measuring with Sense (several people on this forum have discovered that)