At first I thought it was a typo...but the math doesn't come out by doubling the amount of bleach to 4 oz either. I first saw the 1 quart/50 gallons ratio in late 80s...then the 2 oz/5 gals began showing up on RV sites in the mid-late 90s. I didn't check the math...just assumed that someone had come up with a more complicated formula. However, all carry the same "these instructions conform to section 10.8 in the A-1 192 code..." etc. So use whichever ratio you like. Meanwhile I need to decide which one to delete.