Hole handicaps don't correspond with how easy or difficult each hole is. They exist only for determining on which holes higher handicap players should receive strokes to compete with lower handicap players.
It makes sense that a moderately difficult, lengthy par 3 without any severe hazards would be the #18 handicap hole. The good player has to play well to make par and will make his share of bogeys, but the lesser player can probably easily make bogey and will make the occasional par. So the scores that each player will make over time will be closer to each other than they will for the #1 handicap hole (where the lesser player will have more blow-ups, and hence will need the stroke he gets there).
It is up to each course to determine the handicap designations for its holes. I know of a course locally that took a bunch of scoring data for an extended period of time, and used that data to set the handicaps for the holes. The holes with the smallest scoring discrepancies between higher and lower handicaps got the highest handicap designations, and holes with the widest scoring discrepancies were the lowest handicap holes (within each nine--front nine has odd handicaps, and the back nine even).