how autocad calc. the center point for the hatch??

dears

 after drawing a triangle then hatched it then removing its boundary , a blue square will appear , its the center point of the hatch , i tried to calc. the center of gravity of the drawing but it was not as autocad done , then i tried to get the center of the surrounding rectangle which fit the triangle (touch the 3 vertices of the triangle)

so how autocad calc. the center point for the hatch??

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