Not familiar with that printer but borderless can be limited to certain media qualities and sheet/roll selections. Actual roll width sometimes is slightly less than the size used in the driver and when the printer measures that and your print page selection is just fitting (with or without borders) the driver may decide that it is safer to turn the print page on the available paper width. More an HP printer experience so it could be a totally wrong comment for your case.
Met vriendelijke groet, Ernsthttp://www.pigment-print.com/spectralplots/spectrumviz_1.htm
December 2012, 500+ inkjet media white spectral plots.