Nutrient Balances

From what I have read on the internet, nutrient ratios that are recommended are different than Cronometer's green-marked guages.

For example:
The recommended Potassium:Sodium seems to around 3:1, but the green guage shows that to be around 6.

The recommended Zinc:Copper ratio seems to be between 8:1 and 12:1, but the green guage shows the optimal to be 12:1.

What is Cronometer using for sources?