I am a complete newbie to this field. When I look up that lat/lon of a place on the web, I see different values reported. For example, for Jamaica Bay, New York, Google shows 40.6057°, -73.8713°. But the web site www.lat-long.com shows 40.6039936, -73.8354124. Why is there this difference?
A more general question is what lat/lon even means for an area/polygon. Jamaica Bay is an area, how do those web sites determine a point coordinates for an area?
For an area, you can choose to take the centroid as coordinates. There are multiple data sources for an area with the same name. Depending of each one, the polygon representing it may vary. These differences can come from historical reasons, disagreement about the area coverage,... So, the resulting centroid may change too.
Think about sea naming and coverage. Where is the "limit" between 2 seas for example? In this particular case, the name refers both to a New York quarter and a sea area (according to Wikipedia https://en.wikipedia.org/wiki/Jamaica_Bay). The tools can't guess.
Sometimes, to describe/summarize your area, you prefer to use a point in polygon (also named point on surface) or pole of inaccessibility and it differs from centroid. See this image for difference centroid and point on surface http://www.h2gis.org/docs/dev/ST_PointOnSurface.png and this article about an approximation of pole of inaccessibility https://blog.mapbox.com/a-new-algorithm-for-finding-a-visual-center-of-a-polygon-7c77e6492fbc
Another difference can be related to coordinate rounding choices.
To expand on @J.R's comments, another source of difference in lat long is the so called "datum shift". Datum here means the parameters used to describe the earth's shape and size etc. If two maps use different datums, the same point on the ground can have different coordinates (i.e. lat long). This difference of lat long is the so called "datum shift".
