Micrsoft flags its own email as spam

I just checked the junk mail box of my hotmail email account, inside I find an email with subject “Imagine Cup – Thank you for registering.‏” from “admin@imaginecup.com”. So I go to imaginecup.com and it turns out that the web site comes under the Microsoft umbrella, that same Microsoft that Hotmail falls under. I find it quite odd that Microsoft flags its own email as spam. I should note that I did acutally register for this email, so it was not quite spam. Althought I do realise (although I’m just guessing) that the spam tag that this email was assigned was probably automatically generated from other users marking it as spam.