Centering with Epson 1270

My 1270 refuses to center prints. For example: On 8-1/2x11 paper, a
horizontal image 8 inches wide yields a 3/16" wider right margin. Top
and bottom are very close. An 8" high image printed vertically has a
4/16" wider right margin, and again, the top and bottom margins are
the same. "Centered" gives better results than "Standard." I've
looked through the manual, and can't find what I've done wrong.
Normally, I can stand a bottom margin a bit larger than the top, but
certainly, the right and left must be the same. What am I doing
wrong? I'm doing some special prints, so I'm in deep ____.

Many thanks for the answer,

Marty L.