I agree, which is why you need to watch the market in your area and not just rely on a realtor. Their business is to sell homes quickly. The 6% commission on the $40k difference is $2400, possibly split between two realtors, but a $37,600 difference for me. In our situation the comps weren't in my neighborhood because nothing had come on the market in almost a year. They weren't even in the same school district for all of the schools, which also made a big difference.
For our current home, Zillow doesn't factor in that we live on a lake. Last lake property to sell recently was a 4 bedroom/1 bath, 1200 s.f., no garage and 80 feet of frontage. It went for $460,000 in less than a week. It was actually listed lower and there was a bidding war (we know the sellers). We have 4 bedrooms, 2 full baths, a garage with a 2nd story, 2,400 s.f. and 100 feet of frontage and Zillow has our "zestimate" at $338,000. Lake property in our area under $500,000 that isn't a seasonal camp is rare.