Displacement from coordinates

[SUBSCRIBE MAPSERVER-USERS Nuri]

Hi,

I created my own map and I inserted my coodinates. First of all I
translated my GPS coordinates to UTM coordinates and then to pixels to
insert it in a data base to place several symbols in my my.

Al symbols have a displacement. As you can see in
www.nusvirtual.com/selectiva (click link to see de map) small yellow
rectangles are in the correct position but blue symbols are the displaced
ones.

UTM coordinates are completely correct, I placed it manually over the map
but when mapserver place them they are incorrect.

Someone could indicate me why appears this displacement?

Thank you.

Nuri