For multiple projects when pasting in a google maps url the lng,lat values returned in the geopoint field are always off by around 100 meters. If you right click the marker in Google Maps the lng/lat values are different than the ones in Prismic. It looks like the geopoint field is grabbing the values directly from the url string - but those are a proprietary coordinate system for street address locations used by Google and are not standard lng/lat.
Eg url: Google Maps
Prismic field returns: 49.8430447,-97.1686739 (you can see these values in the url above - but those are not lng/lat)
Right clicking in Google Maps: 49.84321764245391, -97.16608827234668
This SO thread explains the difference: Google API giving wrong latitude and longitude values - Stack Overflow