Great question! Property values are estimates based on a variety of factors and sources. Not all companies have access to or use the same sources. This is especially true of homes that have been upgraded or remodeled since the last time they sold. The online sources cannot take this into consideration and hence, their estimate is based on lot size, sqare footage of the home, neighborhood, etc. Conversely, if a property has deteriorated since the last sale, online sources will not know this. Another aspects of a sale that an online source cannot calculate is the reason a home sold for a certain price, what negotiating and concessions were made.
However, Realtors will!! Always best to check with a local realtor to discover what the actual, most current value is for a home.
Hope that helps! And if I can be of assistance, please let me know!