As I understand it, some studies don’t distinguish low-volume drinking from not drinking. Grouping non-drinkers with low-volume drinkers will bias that group to look healthier.
In addition to this, some studies don’t include younger individuals. I suppose this biases the study outcomes towards older-age related effects while neglecting younger-age effects. An example of an older-age effect could be an interaction between alcohol and a blood thinning medication or heart disease. An example of a younger-age effect could be excessive drinking combined with behaving like a complete buffoon