I meant, how strong is the strongest match in the mystery group who is not also in another group? So a first cousin is too close of a match.
With only three groups on the mothers side, one or both mystery groups could be the fourth group. My mother's grandfather was the only one of his siblings to have children with living descendants. And in fact none of his father's siblings have living descendants either. So my mother's group that would correspond to her grandfather splits into smaller, weaker groups corresponding to that grandfather's maternal and paternal sides.
Or if the mystery group matches are quite distant, they could be on the same line as one of the groups you already have, but just not match people in the group you already have because the relationships are too distant.