The reason that the students are showing up with the different parent's phone number is to display any unique entries. This can allow for multiple points of contact using the information found in the directory.
If a relationship has a phone number shared with Parent A, it would not typically display the phone number again as it already is displayed under the Parent. In this case, the students share a phone number with Parent A and with Parent B. Because Parent B's number is different than Parent A's, the students would display the different number when viewed in the directory as an alternative means of communication.