Tucked under edges rails
give you the full board width
when schlogging (going slow).
When you are up and planing (going fast) you are riding a bit narrower contact width because the rails are tucked under their edges.
When measuring the width of the hull at one foot off, make sure to measure the outline width, i.e., the full hull width
at 30 cm, not at the hull's bottom tucked under the edges width (http://www2.swaylocks.com/node/1028157?page=1