ST_Area documentation states that "area is determined on a spheroid with units in square meters" when using geography as argument, or when setting the 2nd parameter as "true".
Usually when calculating areas we can reproject a geometry to an "Albers Equal Area", and these projections may have different source latitude and longitude depending on the region we are working. For instance
+proj=aea +lat_1=-2 +lat_2=-22 +lat_0=-12 +lon_0=-54 +x_0=5000000 +y_0=10000000 +ellps=GRS80 +units=m +no_def.
On the other hand ST_Area does not state any source lat or lon or skews. Are those values dynamic? Or are they "borrowed" from the projection of the geometry? (in case, for srid 4326 would be lat=0 lon=0 x=0 y=0)