[Proj] EPSG:28992 to EPSG:23031 conversion

[bartvde at osgis.nl]

Hi list,

when I use PROJ.4 to do a conversion between EPSG:28992 (the Dutch
national grid) and UTM zone 31 ED50 I get:

cs2cs -f %.9f +proj=sterea +lat_0=52.15616055555555
+lon_0=5.38763888888889 +k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel
+units=m
+towgs84=565.2369,50.0087,465.658,-0.406857330322398,0.350732676542563,-1.8703473836068,4.0812
+no_defs +to +proj=utm +zone=31 +ellps=intl +units=m +no_defs
19465 394814
530108.861080788        5708613.329898705 -150.818087836

If I use the coordinate calculator (Windows only sorry,
http://www.rdnap.nl/download/setup%20cc41.exe) which is an official tool
in The Netherlands, I get:

Northing 5708615,754
Easting 530199,479

so especially the difference in easting is too large.

Is there a logical explanation for this difference? TIA.

Best regards,
Bart