I found this on the web last night and I think it explains it....
'A very frustrating turn fighting with the margins in trying to get a 12x18 inch image centered on 13x19 inch sheets. He contacted Epson tech support and received the following response and discovered some limitations to the way the 4800 prints due to its design.
The Stylus Pro 4800 has a cut sheet margin limitation of .116(3mm) inches top, left and right and .55(14mm) inches bottom. So your image will need to be small enough to fit within a 1.10 inches for a leading a trailing edge and .232 left and right. The only way to get a truly even or "full bleed" margin is through the use of roll paper. The hardware is incapable of holding the paper and printing to the leading or trailing edge of the media. Unfortunately, due to the volume of output that was expected for the 4800 it is was designed to always have the .55 inch margin along the bottom of cut sheet. The printer hardware needs that much space to grip the paper allowing the proper feed step to produce Epson photographic quality prints. The best way work around the margin limitation is through the use of roll paper.
Follow this key sequence on the front panel of the printer:
1. Press menu button twice.
2. Press the down arrow four times till you see "Paper margin"
3. Now press menu once, the selection should show "T/B 15mm"
4. Press the up arrow until you see 3mm then press "menu".
5. Now press pause and the printer will go to read
The .55 inch margin for the trailing edge of the print is because of the fan units used by the 4800 to move the paper along the print path. No rollers are used after the paper is pulled from the tray and into the print path and this avoids a problem that afflicts some consumer printers of "pizza" roller tracks on the printed surface (I saw such tracks on occasion with my old Epson 1270). Epson giveth and Epson taketh away.
I haven't had issues with the margins with my own 4800 except for trying to print four images centered on letter sized sheets, which I don't do too often, so I'm not that critical about it. I also don't print on 13x19 paper very often either, as it's an oddball size for me, as I prefer to print traditional portrait sizes such as 8x10, 11x14, 16x20. I also like to store my prints in Prat portfolio-style leather albums and these max out at A3 size (12x16).
So, if you wanted a dead centered image, size your image to fit within the .55 inch margins on the leading and trailing edges, or use roll paper'
S
