Also, remember that these are phone activations, not new users.
So every time an Android user gets a new phone, that's another number.
Also keep in mind, these polls aren't counting activations with their wireless carriers, they're counting Android activations on Google's servers. Let's say a poor soul is having issues with this Nexus S 4G and he decides to do 3 hard resets in one day, each time he "reactivates" his device with this Google account, it shows an a new activation on Google's servers even though it's really not, so there's 3 activations right there (with the same device).
Also factor in everyone that roots their phone and does a hard reset so they can have a clean install, when they "reactivate" it registers as a new activation on Google's servers.
Taking into account the above and combine that with the saturation of low to high end Android devices that are sold and activated each day, I'd assume that 550,000 is a probably a legit number or pretty damn close to one.