It might be a gender issue, it might be an age issue ... it could be ... a region issue, or something else that affects that response and gives you these 2 subgroups, which is why in a data set we don't just record only the outcome of interest, we record a lot of other variables too, they may not be the main response variable but they might help to explain features that we see in our response, we might have to control for or account for age or gender in our analyses if we see such a picture like this. 