Because you're doing something wrong.
Seriously though, without all the details of how you're setting up the print and what the print is, it's nearly impossible to say for sure.
My first guess would be that your margins are uneven (the default for most printers is to give a larger bottom margin). I know on epson printers you can click a toggle to "minimize margins", which gives you smaller and even margins all the way around the image. If it's being printed based on the printable area's center and not page center, then this would fix it.
Could actually be positioned wrong due to the image not being set up correctly in the document itself.
It could simply look off center visually but not be off center mathematically.
There could be extraneous image data you aren't seeing but that is being printed that is throwing it off.
Basically, without a little more info, I'm just guessing.