Most of my knowledge regarding this is with regards to pets. But you'd be shocked how many people give their pet 1 syringe full instead of 1 unit. (So many small animals given a 100x dose of clavamox...) Even when you mark the syringe with a line in sharpie and show it to them directly and explicitly.
Half the people on Reddit can't even figure this out using the peptide calculator.
21% of American adults are considered functionally illiterate. 50% read at or below the 8th grade level. I'm not saying that makes them stupid (public education is a hot topic I don't really want to get into) but I'd expect that 50% to struggle with the math involved here, or to follow directions given with the vials.