Why do I get correct area and intersect area when use wrong projection?

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I need to calculate areas and intersection areas for polygons (some real geographical objects like lake, city, country, e.t.c.). Polygons located in California, New Zealand, Russia.Anadyr, Sweden

All polygons are in WGS84.

What I did using GeoTool java API:

1. Project all polygons using EPSG:3488, EPSG:NAD83(NSRS2007) /California Albers and calculated areas and overlap areas.
2. Did the same using World_Mollweide and World_Eckert_IV
3. Picked "local specific projections" for polygons from California, New Zealand, e.t.c.

I assume that #3 is the most accurate result, since I pick projection which covers polygon area

Result:

'#2 showed the worst result comparing to #3

'#1 and #3 areas and intersection areas difference is less than 0.1%

Why? I pick absolutely wrong projection EPSG:3488 (California) for polygons from Sweden and get pretty the same areas and intersection areas?



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