Thank you!
My acceptance letter states, "If you do decide before [4/15] that you will not be joining us at Penn, we would appreciate it if you could please inform us as soon as possible. We have a few admits who are on a waiting list for funding allocations, and we cannot complete their offers until we have clarity on our overall enrollment numbers." It sounds like the committee has a list of students that they are hoping to admit, but that they don't have funding for. It's very possible that they are withholding those notifications until a later date, when they have a better idea of who from the "first wave" will be committing.
I will most likely be declining Penn, as I've also been accepted to a better fit program and offered extremely generous funding. I am going to be withholding my final decision until after visit days, but I will let them know as soon as possible should I commit elsewhere!