First of all, the term "Droid" refers to a model distinction (see silly commercial of robot-digits punching screen on phone), not the OS, and only represents a very small percentage of devices running the Android OS.
Also, the graph you are referring to is of recent purchases, not total current users. Total current users for the iPhone accounts for 28% of the smartphone market in the USA (that's four models in all...iPhone, iPhone 3G, iPhone 3Gs, and iPhone 4) compared to 19% owned by Android OS devices (several dozen different models, inclusive of "Droid" badged models).
While the Android OS sales are up as of the last 4-5 months, that is to be expected with the # of releases of different Android OS devices in recent months compared to one for iOS devices.