Supplements are an unregulated market so doses can vary wildly, even with melatonin.

‘A study published in 2017 tested 31 different melatonin supplements purchased from pharmacies and grocery stores. For most of the supplements tested, the amount of melatonin in the product did not match what was listed on the label.”’

Melatonin Natural Health Products and Supplements: Presence of Serotonin and Significant Variability of Melatonin Content https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5263083/