Why is IBM's female CEO paid less than her male predecessors?

Now I'm sure there's a perfectly logical explanation (ahem), but Footnoted's Michelle Leder spotted a recent filing that had IBM CEO Virginia Rometty getting a base salary of $1.5 million, which is $500,000 less than what former CEO Louis Gerstner was making in 2000 - and $200,000 less than what her immediate predecessor, Samuel Palmisano, was making in 2011. Not that $1.5 million isn't a handsome wage (and that's not including bonuses and perks), but it isn't what the guys were making.

Granted, base salaries rarely tell the whole compensation picture and usually only make up a small piece of a CEO's overall compensation, as Theo Francis footnoted when Palmisano stepped down last year. Nor to we expect Rometty to turn to the Lilly Ledbetter Fair Pay Act, which President Obama signed on his first day of office in 2009. Still, if women like Rometty aren't even being paid what their male counterparts are for the same job, what hope is there for the rest of us?