thanks very much for the reply, and your time. I've attached a VS2015 .NET 4.5.2 console app project that removes all but the essentials. GetAreaTest.zip (1.7 MB)
In Visual Studio, build, then run it with F5 and it will print out the area in UTM zone 13N sq meters of each of the 4 features in the feature collection in the file. Compare those results to my earlier screenshot of from ArcMap/QGIS and you will see that the area of feature ID 2 differs by a little over 1 sq-m, and feature ID 3 differs by ~5 sq-m, while features ID 1 and 4 agree within a few hundreths of a sq-m. Example output:
Feature 1 area: 173167.9921875 sq-m.
Feature 2 area: 233988.7578125 sq-m. (should be 233989.9936)
Feature 3 area: 322125.1875 sq-m. (should be 322121.6224)
Feature 4 area: 111790.375 sq-m.
The reading of each geojson feature from the featurecollection boils down to very simple: I use Newtonsoft.JSON to parse nodes in the file and loop over the contents of the "features" JArray, and use ThinkGeo.MapSuite.Core.Feature.CreateFeatureFromGeoJson() on each feature JToken.
I've validated that geometries in this geojson featurecollection are correctly wound (by geojson's interpretation of the right-hand-rule), and anyway it isn't like there is any hole in features 2 or 3 that are of a size to explain the difference I see.