It actually should go up based on the delayed retirement credits.
If you retire before age 70, some of your delayed retirement credits will not be applied until the January after you start benefits.
As of December 2011, she had a bunch of delayed credits (I could count, but the number isn't that important for this discussion). Her December benefit would include those delayed credits. Her January benefit would include those plus 11 more for Jan 2012 through Nov 2012.
Now here is the trick. They don't actually get around to paying them to you for a while. See a thread where nisiprius and I go around about this issue. What you see is what she is going to get paid, not what her actual benefit is, at least I think that is what your letter says.
None of which explains her numbers. A phone call might help. They will be open on Monday. They might even be open today (I forget - nope, see below). You could have called yesterday, if yesterday was when you got your letter (unless you didn't open it until after they stopped answering the phones).
If you cannot handle your business through our automated services, you can speak to a Social Security representative between 7 a.m. and 7 p.m. Monday through Friday.