The FC levels for SWGs have been determined by trial and error. They appear to work for most people. We don't know why the level is lower for SWG pools than for other pools, all we know is that it works. I have heard a couple of different theories, but none of them are completely convincing.
Some pools require higher FC levels than the minimum that appears in the chart. That is why we express it as a range, between the minimum and the target levels. Also, keep in mind that you should test the water in the early evening, which is when the FC level is normally lowest, to be sure you are not going below the minimum at that time.
When you have large storms, it is easy for your FC level to go below the minimum for a while during the storm. if that happens, you can get algae at that time. SWGs tend to fight with algae, such that the algae never really gets going but also never really takes over. You need to make sure the algae is completely killed, by carefully shocking your pool, before you go down to the low end of the recommended range.