Juul uses nicotine salts, where as standard vape juice is just freebase nicotine. The salts are said to be 'milder' or easier to consume.
In the US, vape juice comes in 0, 3 and 6mg/mL doses. Juul does not publish nic levels, but its probably closer to 48+mg/mL.
Most vape liquids I've seen come in 0-24 mg/ml (in 6mg increments, except for 3).
Apologies, i'm unable to find the article i read before, but googling "nicotine salts" should get you on the right path.
To start, this advisory mentions nicotine salts in JUUL pods toward the top of page two.