פירוש על קידושין 129:2

The Talmud solves the problem by saying that the younger group of daughters consists of only two. There is no middle daughter, for had there been one, we would have had no doubt about her being called “eldest.” And this is evident by the fact that a middle daughter is not mentioned in the mishnah.

The problem is that if there is a middle daughter in the first group, the older daughters, she too is part of the doubtful group, for she is older than the second group of daughters. But the mishnah does not teach her? So maybe the fact that a daughter does not appear in the mishnah does not matter, and there might be a middle daughter in the second group.