A couple of times a week, Nick Spencer checks the value of his four-bedroom house in Haddon Heights, N.J., on Zillow. He has no plans to move, describing the town, located about 10 miles from Philadelphia, as “Americana at its best,” and his Cape Cod style home as “a labor of love.”

Yet there he is, clicking on Zillow every few days to see what the house he bought for $399,900 in 2006 is now worth. The last time he looked, the Zestimate — a Zillow algorithm that not only calculates current values for 110 million homes, but also predicts what they’ll be worth in the future — pegged Mr. Spencer’s home at $503,744. A little green arrow showed it up 1.7 percent from a month ago.

Mr. Spencer thinks it’s extremely unlikely that anyone would pay anywhere near that much for his house, charming as it may be. A neighbor down the street just took his house off the market after two years, even after dropping the price by more than $100,000, to $369,000. Zillow has that house pegged at $447,000, and rising.

Mr. Spencer blames location for the discrepancy. He and his neighbor live along the border of two other towns, including Haddonfield, where home prices are much higher, a fact that might skew Zillow’s algorithm. The numbers might be divorced from reality, but that doesn’t stop Mr. Spencer from tracking them.