It only sends the WOL magic packet when it is woken from it's own sleeping state by hitting the power button, as Roger said above:

When you hit the power switch (to resume, not from hard power off), the Receiver will broadcast the necessary magic to wake up the PC that it originally got its firmware from.

So it does know the details of the PC it booted from, because it booted from it, got put into it's sleep state (where the screen is blank but the processor is still running the Linux kernel and the receiver client) and then awoken from it.

On a reboot (for example when you first power it up at the mains) it doesn't send a WOL magic packet, because it doesn't know where it's server is yet.