Export Map - Pixel Length and Width

I have a workflow that I'm using to feed a template report. Everything works fine, but setting the ExportMap's activity for the image length and image width won't keep

centred. Rather the gap on the left side is larger, while the gap on the right side is narrower.

/customer/servlet/servlet.FileDownload?file=00P6000000elznsEAA

If I increase the image width alittle, the right hand side goes beyond the map neat line, while the left hand side basically stays the same.

Any clues as to why this is happening?

Walter