Likely, what we're seeing here is the result of a few very well-publicized OTA updates for owners of Samsung Galaxy S2 and HTC Sensation family of devices. However, the updates remain elusive for the majority of Sony Android phones, as well as popular older models like the HTC Desire and Nexus S. The Nexus S, at least, received ICS last week, but it doesn't work yet on the Sprint version of the phone.
Gingerbread (Android 2.3) still reigns supreme, running on 63.7% of Android devices, but its growth has finally slowed down in recent weeks.
According to Engadget, it took about a year for Gingerbread to achieve market dominance, so it could be the same with ICS, Certainly developers should think twice before creating apps with features that'll only work with ICS-enabled smartphones. Hopefully Sony, HTC and Samsung will roll out the updated Android software on their older phones soon!