St. Thomas Aquinas explains this in Summa Theologica I q. 93 a. 4 ("Whether the image of God is found in every man?") ad 1:
The image of God, in its principal signification, namely the intellectual nature, is found both in man and in woman. Hence after the words, "To the image of God He created him," it is added, "Male and female He created them" (Gn. 1:27). Moreover it is said "them" in the plural, as Augustine (Gen. ad lit. iii, 22) remarks, lest it should be thought that both sexes were united in one individual. But in a secondary sense the image of God is found in man, and not in woman: for man is the beginning and end of woman; as God is the beginning and end of every creature. So when the Apostle had said that "man is the image and glory of God, but woman is the glory of man" (1 Cor. 11:7), he adds his reason for saying this: "For man is not of woman, but woman of man; and man was not created for woman, but woman for man." (1 Cor. 11:8)
tam in viro quam in muliere invenitur Dei imago quantum ad id in quo principaliter ratio imaginis consistit, scilicet quantum ad intellectualem naturam. Unde Gen. I, cum dixisset, ad imaginem Dei creavit illum, scilicet hominem, subdidit, masculum et feminam creavit eos, et dixit pluraliter eos, ut Augustinus dicit, ne intelligatur in uno individuo uterque sexus fuisse coniunctus. Sed quantum ad aliquid secundario imago Dei invenitur in viro, secundum quod non invenitur in muliere, nam vir est principium mulieris et finis, sicut Deus est principium et finis totius creaturae. Unde cum apostolus dixisset quod vir imago et gloria est Dei, mulier autem est gloria viri; ostendit quare hoc dixerit, subdens, non enim vir est ex muliere, sed mulier ex viro; et vir non est creatus propter mulierem, sed mulier propter virum.
cf. Popik Philosophy of Woman of St. Thomas Aquinas, pt. 1 "The Nature Of Woman", § "Essential Equality Of All Humans"