I would think that given the wide variation of pool conditions (different climates, trees vs. no trees, bugs, rain yada yada yada) these numbers probably work well no matter what. If you tried to build a table that took all of these factors into consideration, it would take a lot of work and it would probably be huge.

I would also think that Ben's table allows for some wiggle room - if you don't get to test for a day or two or have more people in your pool one day or there's a rainstorm, whatever, then it won't cause your pool to go south whereas if you keep it right at the stable point then any little disturbance could put you over the edge.

Also it seems to be one of Ben's goals to make pool care easy for people so rather than have people figure out their minimum chlorine level, here's a universal guide that is reasonable and works. Given that 4 ppm of chlorine is considered acceptable in drinking water, these numbers don't seem like much of a concern.

Peter