That being said, I wouldn't say 10% is a super high overestimation. On my street are two identical houses and one sold for $150k while the other sold for $180k, even though they're literally identical other than the color of the siding.
The number that really matters is the appraised value. Zillow had my house at $110k when I bought it and the buyers were asking $140k. It was a surprise when the appraisal came back at $145k. If Zillow was right, the bank would never have given a loan for $30k over the value of the house. But there was no way for Zillow to know the amount of work the previous owners put into the interior of the house.