Commentary for Kiddushin 128:8

A father cannot marry off his daughter once she is a bogeret, a girl of majority age. So this mishnah is too obvious—obviously the bogeret cannot be the girl who is betrothed.
To make the mishnah less obvious (and therefore not extraneous) the Talmud suggests that this was a case where the bogeret appointed her father to accept her kiddushin on her behalf. The mishnah rules that if the father does not remember which girl he betrothed, we can assume that he did not betroth the bogeret. The assumption is that since if he marries off his younger daughters, he would receive the betrothal money, he must have married off one of them and not the older daughter who appointed him as an agent.