The exposure constant is the numerator in the film speed equation. While the speed point is chosen because of it's relevance, it might be desirable to place the exposure at a different place. Any number other than one actually determines a point other than the speed point for the exposure placement. What if the ISO speed equation was 0.4 / Hm? This would offset the exposure by a stop.

BTW, I don't see the ZS EI as being designed for the discriminating taste of the photographic elite. It's just the safety factor from before 1960. All anyone has to do is take the ISO film speed and cut it in half.