There are three major providers of commercial map data, Navteq, Teleatlas and Google. Navteq is owned by Nokia and Garmin licenses their map data for City Navigator as well as their 24k topo series. TomTom owns Teleatlas and provides (some?) of the data for Apple Maps. Google stands alone, although at various times in the past they purchased data from both Navteq and Teleatlas.
They all use a variety of methods to obtain their data and there are bound to be some differences.