You may be surprised to learn: unlike with food products, “MAY CONTAIN” is not a warning, and it is not related to cross-contamination or allergens. It's all about the pigment that creates different colors in makeup.
In makeup, different colors in the same product line will share a base formula, but will contain different pigment blends to create various shades and finishes. These colorant ingredients that are not present in every shade are the items you see listed under “MAY CONTAIN.”
Separating these colorant ingredients into their own section is important for INCI list accuracy; without making it clear that they are not present in every shade, the ingredient list would be promising something that the product isn’t actually delivering.