Algae forms a protective coating that has to be brushed off for the chlorine to do its job. Its easy to remove that protective layer by brushing walls, floors etc. then the chlorine in the water can easily move in for the kill.
The problem with hidden algae is that it is in places like inside of your ladder, skimmer flaps, drains, and other places that do not normally get brushed in the normal course of pool care. Its also usually not enough algae to cause you to fail the OCLT, so people think they are algae free and stop the SLAM and then the algae returns a few days/weeks later.
I'm not saying you have hidden algae, but its certainly worth taking a look at that ladder, especially if there is a pile of dead algae near there. Hidden algae is very frustrating, but its awesome to see the eureka moment when someone finds it and gets rid of it so they can end the SLAM and return to enjoying their pool.