The reason you have pivot values for x, y, and z is because your mesh has transform values of x, y, and z (values other than 0). The only reason I can think of for your mesh having a gap like that is that you have at least 1 vert out of line.
Check the vertices along the center edge to see if one of them is not lined up with the rest.
edit: The pivot values are most likely not the reason for the gap. It's normal to have pivot values if the geomety that you are mirroring has transform values other than zero. You probably just have a stray vert or face or something. somewhere. Go to a front view and look at your geometry in wireframe mode to see if you can see anything like that before you mirror.
Don't be satisfied with what you can do but rather strive to do the things you can't do!
Last edited by Perfecto : 15-10-2009 at 02:18 PM.