Apple are aware of both your telephone number and your Apple ID being used for iMessage. By switching iMessages on, Apple are told to deliver messages sent to your number via iMessage instead of a network text (SMS). Apple demonstrate they know this when you go to send a message to someone and the 'send' button turns from green to blue, indicating iMessage is active on the receivers phone.

Whilst I don't know for sure, I assume this will be the method used to redirect messages sent to an iMessage registered phone number to an Apple ID running on OS X/other iOS devices.