Website creator/maintainer here. Happy to try and diagnose the error if you elaborate on what the discrepancy is (tree, building, hill, etc)...
Hi there! My office window faces east and this morning, your map showed shade from the building across the street (correct), as well as shade from my building on the lawn below my window (incorrect).
Very cool tool! It's generally accurate, but, there are spots showing shadows where there are no trees or anything at all to cast a shadow, and missing several spots where there are actually trees.
It's missing a 40' tall ash, a 40' tall maple, and all four 30' tall blue spruce. So they're not exactly small trees. Our septic field is showing as in shadow all day even though there are no trees near it. That shadow doesn't move at all like the rest so I'm not sure what it's detecting.