It could be a legitimate signature. Introduction of subtle differences is a good security trick. Signatures are not even supposed to be your name, written by hand - they are supposed to be unique handwriting that is tough for a stranger to copy correctly. My own signature, for example, does not exactly spell out my name - it begins with characters, but then quickly devolves into something completely different.
Yet another aspect is that signatures are formed in adolescence, when young people start needing them. They could prefer a different form of their name then. Obama's grandmother was born in 1922, and her signature was probably fully defined by 1942. It was 70 years ago; many styles and fashions changed since then.
Of course, it should be trivial to find a few papers with her true signature and compare. She signed thousands of personal checks, for example; but she worked as a VP of a small bank - can you imagine that she didn't have to sign a hundred papers per day at work?
Re signatures..
Haste plus tons of things to sign equals illegibility (think doctors, celebrities)
So yes, quite possible...
And examples of SADOs moms signature could be found among those FOIA docs of SADO.