Whats going on here with Chlorine readings

Stock is Liquid Chlorine at 13%.

To get 5ppm reading on two different brands of dipsticks in 20 litres of water, I need to add 6ml of the 13% Chlorine concentrate.

This is way above what any calculator says with 13% stock why would this be happening, calculator says .8 of a ml where I need 7 times that, I use rainwater and have had a good dumping of rain lately and the water has a slight tint from the tannin, could this be why I need so much base to get a reading. If I had only one test strip I wouldn't believe it but two different strips are giving the same reading.

Any body running chlorine can you smell it in a cup of your mix water.

Yes you can smell chlorine in the final nutrient solution.

Ya I get the same result of 0.8 ml mL to 20 L dilution. I could see organic loading influencing this a little but not by a factor of 700%. Try a clean water soution to confirm your colorimetric test strips are accurate.