Apparently, because the Japanese character for ichi is one horizontal mark, ni is two horizontal marks stacked, and san is three horizontal marks stacked, if the instructor were to write the certificate using the character ichi, then an unscrupulous student could go around the corner, take his own brush, and instantly promote himself to sandan by drawing two more strokes. I don't know if this ever happened or if this story is apocryphal (why would you teach someone who would do that anyway?), but it's a cool and interesting story.
I think that shodan (meaning beginning rank) conveys the sense of the true meaning of the rank better than does ichidan (first grade). Shodan is a beginning. You have just gotten to the point that you understand the superficial foundations of the art. You have come to a point that you can see the face of the art. You are competent to practice the basics of the art. You have only just begun.