Transformation between EPSG: 32632 and EPSG:32633 - problems with area values

[Antonio Valanzano]

I have downloaded a shapefile from ISTAT website with the
administrative boundaries of italian municipalities (Com01012026_WGS84
with EPSG:32632).

When I calculate the total area for my county (cod_reg = 17) I find a
difference between the value of the area calculated with the original
coordinates and that calculated with the coordinates transformed from
EPSG 32632 to EPSG 32633.

Here is my sql script with the results

SELECT
SUM("shape_area")/1000000 as sum_shape_area_column_kmq,
SUM(ST_Area("geometry"))/1000000 as sum_area_geometry_32632_kmq,
SUM(ST_Area(ST_Transform(geometry,32633)) / 1000000)as
sum_area_geometry_32633_kmq
FROM "Com01012026_WGS84"
WHERE cod_reg = 17;
-- 1 row
10073.185398 10073.185398 9986.345686

there is a difference of about 87 kmq.

Could someone explain to me why there is this difference if I have
only changed from time zone 32 to time zone 33 within the same datum
WGS84?
The geometries should not have been changed apart from a different
shift in the origin of the x coordinates.

Thanks in advance
Antonio