Called UMA (Unlicensed Mobile Access), the technology is designed to provide better call quality indoors (where your cell shouldn't otherwise work, natch) by switching over from a weakcell signal to a stronger in-home WiFi signal.
One of the complications with the MicroCell is that in order to avoid interference with local cell towers, it adjusts how strong the signal that it generates in your house is based on how weak your actual cell signal is at the time of initial activation.