There really isnt any mystery.
I claimed single 1 the whole year but got married in April. My wife doesnt report any income.
I grossed $108,000. I also bought my house in 2012 so i had my closing costs, interest, and mortgage insurance to write off. I also have student loans and can write off up to $2,500 in student loan interest.
I contribute 9% to my 401-K (4.5% traditional / 4.5% roth) so that helped lower my AGI.
I also had some other non-profit contributions as a write off.
I changed it for this year and now claim married 5 because i dont want the government working off my $ anymore. Id rather owe them at the end of the year. But even at married 5, i still might get some back as I am having a child in 2013.