This is a function of the printer driver. It is why a lot of the drivers to to ensure that the entire surface of the paper will get covered. I don't believe there is anything you can do about it.
This is a built-in feature when printing borderless. The image is expanded by design to make sure that there are no white edges on the paper due to tolerances in the paper feed in the printer or in the nozzle travel within the printer. You can reduce this some by changing the expansion amount in the printer dialog by selecting Min or Mid in the Page Layout Settings area. By default it is set to Max. Mine works fine with Min; you need to test a print on Min. If you see a white line on one of the print edges, try Mid.
Have that printer, can't print borderless either in Photoshop so I suspect it's a driver issue or something. I'm on OS X if that matters.
It took me a bit of fiddling but I was able to print borderless images from Lightroom 4.4. I'm using Windows 7.
In Lightroom, on the Print tab,make sure that your margins are set to 0 and the your cell size sliders are all the way to the right, in my case height of 4.00 and width of 6.00.
In the Printer Properties, set to Borderless.
In Lightroom there is a couple of pixel white border on the right of my picture, evident in the first screenshot, but it doesn't appear on the actual print. I hope that this helps somebody.