Is all breast milk made equal? Is it not possible that some mothers milk is deficient for some reason (diet, genetics, health issues etc)? It's not usually tested after a child is born.
Diet before, during and after pregnancy can affect the nutritional composition, and it can be low in vitamin D, iodine, iron, and vitamin K. Vitamin D and K are usually supplemented (K is given at birth).
Some women (around 30% iirc, can't find the number atm) do not have B. infantis because of low gut diversity, which is really important for infant gut health and long term immunity. Infants are usually exposed during vaginal birth, so babies born by c-section are at risk for not having B. infantis colonize their gut. Breastmilk can be a source of B. infantis.
Some communities with heavy environmental contamination have had lead levels as high as 41.1 ppb [1]. Baseline heavy metal content for most areas tends to be more in line with formula.