OK, thanks guys, I thought the SWG chart was based on the fact that the FC levels are held stable (for the most part) through the day by continuously adding chlorine to the pool, unlike the manual method of adding chlorine where it's added just once a day.
The SWG chart is based on observational experience. We don't really know the true reason as to why a lower chlorine level works OK for SWG pools. Some of it may be the more consistent dosing so that it doesn't get too low but some of it may be some super-chlorination of a portion of the pool water going through the cell. I suspect it's a little bit of both though more of the latter, but to play it safe we recommend the normal dosing chart. Of course, you are free to experiment with what works in your own pool and report back to us what you find.