Here's the abundantly obvious answer to your question:
Either you're physically doing something to wake up your PC, or something in Windows on your Mac is sending out the "magic packet" to wake up your PC. Or, maybe your PC wasn't really asleep, and the screen is just coming on because it sees something new on the network.
I have absolutely no idea what the real answer is, but it's probably going to be describable by one of the above.
lol. Well I am definitely NOT physically waking it up, I am no where near it usually when it happens. Like, across the room. And it is definitely really asleep, as I put it to sleep and all the lights go out, etc. So, magic packets must be it...