Is it possible everyone on my block built their fence wrong?

A lot of times the GIS maps do not represent a survey and only contain information only required for government purposes.Some times the maps show your property for example as 100x120 when your property is only 80x100. The GIS maps show public right of ways as well. A lot of citys require building permits and will tell you where you can build and where you can't. I wouldn't worry too much about it. Sounds like an error in the software.