Most signings on "Signing Day" are bunk and are strictly used for publicity by schools, coaches, parents, players, media, etc...
Technically, only players receiving athletic scholarship money are obligated to actually sign a Letter Of Intent. The NLI is in essence a contract for I believe 2 semesters that locks the student athlete into the school and outlines the financial commitment of the institution. I'm pretty sure Service Academies are not considered scholarship schools and do not use "actual" NLI. I think IVY League schools also do not offer athletic scholarship. However, both SA's and IVY's have "recruited athletes".
Recruited Athletes still must gain Appointments. My son actually signed his Appointment during his school's signing day festivities. I assume that coaches only have so much input in to who and how many Appointees they can assist getting in.
I hope this helps. I am certainly not an authority on this. I tried to say "I think" and "I believe", which is another way of saying "I don't really know".