Even if the print is setup in LR full bleed borderless, unless you choose marginless in the Epson driver dialog the driver will put a margin on the print. Still not sure why yours is off to one side unless the paper is not feeding in evenly. I don't do a lot of marginless work, but it never fails when I make the first one from LR, I will forget to check the box in the driver.
You should be able to print marginless on a sheet width wise, but as already mentioned, due to the way Epson handles sheets, you will always have a 1/2 inch border at the top of the print. (running length).