[Proj] Question about Italian conversion

[Massimo De Marchi]

Hi,
I am a new user, and was performing image conversion with gdalwarp 
between old Italian c.s. (Gauss Boaga) and WGS 84.

I found there is a shift (around 100m)in the conversion. I tested with 
cs2cs and I have the same issue.
The coordinate conversion is the same using both EPSG 3003 or 26591, or 
using the proj4 string taken form spatialreference.org

Starting with 1641163 East and 4997349 North I get
10d47'40.422"E  45d6'52.824"
in place of:
10d47'39.5"E    45d6'55.2"

What am I doing wrong?

Thanks in advance,
Massimo