Having never had a manager with any clue about maths, I found the golf analogy works better:

"Why can't you hit a hole in one with every golf shot? You know exactly where the hole is, you know how much the ball weighs, you know the length of the club, you know how hard you need to hit it and in what direction. So why isn't every golf shot a hole in one? Because, obviously, you may have miscalculated how hard and at what angle you should hit the ball, you may not have the skill to hit the ball exactly right, and any amount of random factors can affect the ball between you hitting it and it arriving at the green. Same for software development"