1) Giving somebody a name usually associated with the other sex can really screw them up. It can also make other people's reactions to them a little screwed up. So I wouldn't assume that Obama's mother always went by Stanley or that everybody who came to know her called her by that name.
Maybe her own mother took her to the hospital, called her "Ann" and the nurses picked up on it, so she signed her name that way, and then remembered that she was supposed to sign it "Stanley". That's a wild speculation I know, but with this topic, everybody's entitled to be wildly speculative at least once.
In adolescence -- and Obama's mother wasn't yet 20 when she had him -- some people are tempted to play with their names. "Ann" may have been a transition between "Stanley" and "S. Ann." Certainly it sounds like a better name for a girl than they do. I'm thinking -- wild speculation again -- being a little rebellious and free from dad for a little while she might have wanted to forget about the name he saddled her with.
2) Trying to fit a signature into a small or irregularly-shaped space can make the signature a little different from what it would be if you had more room. Some people will sign right over the writing on the form, others adapt their signature to the space that's left.
3) It looks like Obamama's signature is written with a fountain pen, and nowadays writing with pre-ballpoint, pre-felt-tip pens looks strange to a lot of people. That varying thinness and fatness may look unnatural and suspicious to people who aren't used to it and create doubts that don't have much foundation.
The Sue named Boy? :o)