Re: Why the difference between FC levels here and manufactur
Full disclosure:
Intex swg, northern clim (short swim season, sometimes lower nightime temps, lower overall pool temps compared to the southern folk unless we go crazy with the solar), CYA of 50 ppm (currently).
I will let my FC run as low as 1 ppm

(poolcalculator.com has 2 a my minimum), not all the time, but sometimes 1-2 times a week depending on temperature. Don't yell at me frog
I should mention these additional caveats, I test FC and CC daily, I run the swg during the day, my pump runs on low 24/7 (unless doing solar, then it is on high), when there is high swimmer load I increase my intex swg on time and on rare occasions suppliment with LC, and my family is very clean (okay this last one is a lie...my wife, who is clean, would not agree that the rest of us are, me and the three boys basically take our baths in the pool during the summer)
/disclosure
They guidelines for levels on this forum are conservative (FC higher), and rightly so. This is the "Trouble Free Pool" site and the levels are based off of empirical data that has shown to give good results to most pools, in most locations, environments, etc. When knucleheads like me

push the limits sometimes/sometime it may/will backfire.
As to why the swg manufacturers list lower levels, I think they are not being conservative, they may be assuming other methods of algae prevention are at play, like algicides, etc. Remember, although swg systems are gaining steam and changing the industry, they are not separate from the whole pool store food chain.