That may be an all-time high, but still less than 5% of the total. Considering roughly half of the U.S. workforce is female, it's not exactly proportional.
However, change is gradual – in corporate America as in other aspects of society. One prevailing, and pragmatic, reason is that comparatively few women executives are actually put in positions that place them in line for the top job.
It's not the president-to-CEO succession that represents a quantum leap up the corporate ladder; it's getting to the echelon below that, which is frequently a senior operational job. Do some members of senior managers and corporate boards have trouble envisioning a female in a traditionally male role? No doubt, but it may be more complicated than that.