If you order an 18"x40" door from IKEA, it'll arrive measuring 17 7/8"x39 7/8". Because our doors are designed to fit in perfectly with IKEA cabinet boxes, we also produce fronts with these dimensions in mind. All our doors and drawer fronts are produced 1/8" smaller than the whole-number measurement given. (We give the whole number measurement to make it easier, since that's what IKEA does too.)
Why the gap? If you were trying to fit two doors next to each other that measured exactly 18"x40", they wouldn't be able to open. They need that tiny space to be able to swing open and shut smoothly without rubbing up against or catching on neighboring fronts.
All parts that are meant to be fixed in place, like panels and toe kicks, are produced to the stated whole-number measurement.