Stabilizing Cl in bleach vs. Cl in pucks

[Poconos]

There are people on this forum that do lose some or all of their CYA at times. Watermom loses it all over the Winter I believe. I lost a little this Winter too. Some believe it's algae or something in the water that causes this to happen but don't know. Recently with all the water falling from the sky here in Northeast PA, and after draining the pool a bunch of times, the CYA went below 30 so am now using pucks to get it back up to the 40-50 range where I like it. I also took the opportunity to use the rain (basically distilled water here) to reduce CH some as I use cal hypo too. Actually I use bleach, cal hypo, and pucks as appropriate.
Al

[waterbear]

There are a lot of 'fariy tales' about CYA floating around pool stores. One that I hear (and it really drives me crazy) is that the CYA is only in the top 6 inches of water so if you want to lower it drain the pool from the skimmer!
This goes against everything I know about the chemistry of solutions. I would really like to know how this myth got started!