Why area of intersection differs from total area of polygon?

Question

I calculated intersection between Layer 1 and Layer 2 using expression below:

area(
    intersection (
        $geometry, 
        collect_geometries(
            overlay_intersects ('layer1',$geometry)
        )
    )
)

And total area of polygon on Layer 2 using: $area

Given the fact that polygon drawn on Layer 2 is completely inside polygon from Layer 1 the area of intersection should equal the total area of polygon but for some reason that's not the case and I don't understand why and how to fix it.

This question is a indirect follow-up to this thread: Calculating area of polygon intersection with condition

Answer 1

As stated in my comment, check Using area() or $area function in QGIS, when the data (polygon) is stored in UTM? for the difference between planimetric area area() and ellipsoidal area $area for more information.