Let me make another wild conjecture. There are two ways to get an increase in benefits:
1) you are in a position where something is supposed to happen automatically.
2) you have to take an action.
From something you wrote, I am guessing this is a 2).
SS doesn't tell you
This is correct. If you go in at 62 and apply, they don't tell you that you would get more by waiting until 66. If you go in at 66 and apply, they don't tell you that you can do a restricted filing and wait to take your own benefit at 70. SS is responsible for 1), but you are responsible for 2). Perhaps your mother was a 2).
P.S. I lie just a little bit. There are some situations where if you ask to do something, they have to tell you some of the consequences. For example, if you withdraw your application, they have to tell anyone whose benefits depend on your having a benefit (in that case, they need the approval of the other party).
1. Explain Effects
Explain fully the effects of consent to the beneficiary whose entitlement will be affected (or the proper applicant on that person's behalf). Inform the person that without his/her consent, the request for a WD will be denied.