I've just spotted that "Just Scan" mode is available when viewing Freeview HD (it's greyed out when on the SD channels). Just Scan shows slightly more of the image (as opposed to using Original or 16:9).
Just Scan is LG's terminology for 1:1 pixel mapping. When this is disabled, the TV will perform slight over-scan on the video image. Basically, the video is zoomed in slightly and the edges are cropped off.
Most SD TV broadcasts have small black borders around the video's edge. Over-scan prevents you from seeing them. This isn't such a problem with HD broadcasts, so it can be turned off to prevent any unnecessary cropping or scaling of the image. Some HD broadcasts may still have a thin black border around the edges. If this annoys you, turn Just Scan off.