Chain lengths

About the second year after owning Music, I put all the anchor chain out on the dock, measured it and painted the markings. Ever since then I have believed that the marks were 5 leagues (aka 30 feet). I have always anchored assuming that was true and that worked until I went out into the ocean.
For the past two years where I'm anchoring in 20+ feet of water, I noticed that my anchor didn't set as well as it should at 5:1 ratio, so I've been using 8:1 ratio.
Today I dumped all of the chain onto the ground and found that instead of 30ft per mark, I have 20ft per mark. This explains why and 20 ft. of depth I had to use what I thought was 150ft of chain to be secure (7.5:1) when in fact I was only using 100ft of chain (5:1), and that is perfectly normal until you get winds above 40Kts.
Oh well, I'm still not sure why I thought it was 30ft per mark…. It is clearly 20ft per mark.